The 33-year-old Hawaiian grimaced and turned away. For a moment, it seemed like he might call it a night. But Penn kept fighting. And the beating continued.
MacDonald showed his championship credentials in a unanimous (30-26, 30-26, 30-27) decision over Penn, a former lightweight and welterweight title-holder. In so doing, he literally treated the future Hall of Famer as a punching bag.
Still the young Canadian rued not stopping Penn earlier.
"It's bittersweet because the job wasn't finished," he said later. "But that's just my outlook to fighting."
MacDonald's approach to mixed martial arts involves hurting people.
"When I look across the Octagon at somebody, it's just another body to me. Legend or not. I'm going in there to hurt him," the 23-year-old Montreal-based fighter said Saturday night.
Job done, as a battered Penn was sent to hospital afterwards.
"At the end of the day, his main focus in that fight was to damage B.J. Penn," said friend and training partner Mike Ricci. "That's Rory's thing — damage. That's all he wants to do, whether he's in a clinch, standing or on the ground, he comes out there to hurt guys.
"He's not there to win rounds. He wins them because he does so much damage. He's not there to play the game and score points and fight these strategic fights. He goes out there, he keeps his game plan extremely simple and just tries to hurt his opponent."
The fight at KeyArena was third on the bill but had a main event feel.
MacDonald was the villain of the piece, booed as the stone-faced fighter walked to the cage to Lupe Fiasco's "Lightwork." The catcalls turned to wild cheers as Penn followed, to the melodic sounds of his traditional entrance song "Hawai'i 78" by the late Israel Kamakawiwo’ole.
The atmosphere was electric. The fight that followed was one-sided.
According to FightMetric, MacDonald had 57 significant strikes to just seven for Penn in the second round, Overall the total was 116-24 in the Canadian's favour.
Consider those numbers and the fact coach Firas Zahabi considers MacDonald "probably the hardest-hitting athlete pound-for-pound I've ever trained" — he switches sparring partners every minute and a half or two minutes back at the gym to save them from abuse — and you have an idea of how much punishment the undersized Hawaiian took.
Zahabi said most fighters would have broken in the face of MacDonald's onslaught, an attack fuelled by brutal body blows.
"We very much trained in body strikes," he explained. "It will slow any man down. B.J.'s got an incredible chin, he's got a head like a brick. His ability to move forward is incredible. It's stunning, actually ... The body is where you're going to slow a man down."
Penn came out of retirement to fight MacDonald, an opponent he hand-picked. UFC president Dana White told Fuel TV that MacDonald had probably sent back Penn into retirement.
Penn is now 1-4-1 since the start of 2010.
"Rory looked better than he ever looked," White said. "The body punches he threw, you don't see punches like that in mixed martial arts. He really put it to B.J. tonight."
Cesar Gracie, head coach to Nick and Nate Diaz, was also impressed.
"He was just too big for B.J." Gracie said. "Rory's a big 170-pounder, he could probably fight at 185. He's a big guy, he's very athletic, he's very strong. It was just too much for B.J. Penn."
Ricci likened the fight to a dance, with the six-foot MacDonald pulling the strings so he could always maintain the proper distance to control the action.
With the five-foot-nine Penn unable to find his range, MacDonald went to work.
After the MacDonald win, another veteran took a beating at the hands of a young gun as rising Swedish star Alexander (The Mauler) Gustafsson manhandled former light-heavyweight champion Mauricio (Shogun) Rua en route to a 30-27, 30-27, 30-26 decision.
In the marquee fight of the night before a sellout crowd of 14,412, lightweight champion Benson (Smooth) Henderson dominated Nate Diaz in a lopsided 50-43, 50-45, 50-45 decision.
It was a mature, impressive performance by the 155-pound champion, who has struggled to win respect since beating Frankie (The Answer) Edgar in two close title fights earlier this year. Henderson showed all the weapons in his arsenal, and then some. In the third round, he essentially did the splits to evade a submission attempt.
MacDonald (14-1) has now won four straight and is 5-1 in the UFC. He has set his sights next on Carlos (Natural Born Killer) Condit, who handed him his only loss in June 2010.
While he introduced himself as a championship contender Saturday, MacDonald has many fans to win over outside of Canada.
His icy take-no-prisoners demeanour — especially against a beloved veteran like Penn — is hard to embrace. Some have dubbed him "Canadian Psycho" for his attitude towards fighting and penchant for elegant clothes.
In truth, MacDonald is more complicated than that.
Intense and very focused, MacDonald has been driven to succeed in the cage since he took up MMA at the age of 14. He had his first pro fight at 16 in Prince George, B.C., with his parents signing a waiver.
Some young athletes are whipped on from an early age by parents. MacDonald is his own master — a young man on a mission.
"I'm not the same person you're seeing right now when I fight. I'm not a nice person," he explains. "I feel like most of the time I'm a pretty nice guy but I have that capability of switching to that other person.''
Away from the gym or cage, MacDonald is not kicking puppies or stealing his friends' lunch money.
"Junk food, candies and sweets will make Rory MacDonald smile," said Ricci. "A good joke will make him smile as well.
"There's not much that makes that man smile," Ricci then added, between laughs. "Me and him, we crack a lot of jokes and we get a lot of laughs in. I've seen him laugh him quite a bit."
A discussion with a Daily Telegraph reporter over suits also made him chuckle following the post-fight news conference.
MacDonald's love for clothes comes partially from the fact that he had one outfit growing up — jeans and a hoodie — and was teased about it. He can afford more these days.
An example of how Macdonald is easy to misunderstand was shown when he did the Ali shuffle late in the fight.
The crowd booed, thinking MacDonald was mocking a fading Penn. MacDonald explained later it's not showboating, just something he does to relax and distract his opponent.
MacDonald also can come across as aloof, as when asked about the boos he got Saturday night. He just doesn't sugarcoat his answers.
"Doesn't matter. I don't fight for them, I fight for me," he replied. "People can love me or hate me. I fight for myself and because I love it. It is what it is."
Despite his success at such a young age, fighting has not always been smooth sailing.
Five years ago, he says he was struggling with focus during a training camp. He was distracted and having problem with his girlfriend at the time.
He won the December 2007 fight — defeating Kajan Johnson for the King of the Cage Canadian lightweight title — then elected to step back from the sport. So he left his native Kelowna for Langley, B.C., stopped training and took a carpentry job.
"I did that for about five months and I hated it,'' he said.
He moved back to Kelowna, and started training again. He won the King of the Cage world lightweight title next time out, in November 2008. Two wins later, he had a UFC contract.
Away from the gym, MacDonald says he is into "regular young guy stuff.'"
"I like video games, I like shopping, I like girls,'' he said.
The Condit fight was a turning point for MacDonald, who became the UFC's youngest fighter when he signed on at 20 in the fail of 2009.
His first fight was a small televised event in Fairfax, Va. in January 2010. Macdonald submitted journeyman Mike Guymon in four minutes 27 seconds.
Then he was hurled into the maelstrom of UFC 115, in his home province at Vancouver's GM Place, against former WEC champion Condit.
"People were going insane," MacDonald recalled in a recent interview. "I never heard that level of noise in a building ... I was super-shocked and it just got me fired up to a point where it was, like, bad. If you watch that fight you could see the intensity that I was bringing and I don't think that was my style. And I paid for it."
MacDonald started strong but faded. The fight was stopped with just seven seconds remaining. Viewers may disagree but MacDonald says he was humiliated
"Because I was just laying there getting beaten on," he said. "My face looked like I was a guy from 'The Goonies' after. I was embarrassed, I was embarrassed about my performance and how I held myself. It did a lot of damage and I don't think I've been the same person since. So I want to get that back."
He moved to Montreal after the loss, to train with Zahabi, welterweight champion Georges St-Pierre and other elite fighters at the Tristar Gym.
Condit, a former WEC champion, is coming off a five-round loss to St-Pierre at UFC 154.
With training partner GSP holding the UFC title, decisions will have to be made soon if MacDonald continues his climb.
Both men have said they won't face off in the cage.
"Me and Georges are not going to fight. ... I wouldn't disrespect Georges or the gym in that way,'' MacDonald said recently.
It may be semantics, but there seemed to be a slight change — at least very temporarily — when he was asked about it after the Penn win.
"I don't know. I'm not there yet so I'll cross that bridge when I get there," he said. "But I don't feel that I need to fight Georges. I don't think it's going to happen, it won't happen. Me and him are friends. He's done a lot for me and I'm very grateful for it, I'm not going to stab him in the back.
And I don't want to wreck my opportunity training at Tristar, they've done a lot of things for me. So I don't know. We'll see. We'll see what happens. There's lots of fights for me."
St-Pierre sent a congratulatory text after the fight.
MacDonald heads to Las Vegas next to support Ricci in his bid to win "The Ultimate Fighter" in Saturday's live finale.