Penn, a former lightweight and welterweight title-holder, has not always maximized his training and it has cost him in fights.
The 33-year-old Hawaiian retired in the cage in October 2011 after a lopsided loss to Nick Diaz. But after mulling over his future, he told UFC boss Dana White that he wanted to return, with the highly rated MacDonald (13-1) as his choice of welterweight opponent.
Penn (16-8-2) said he told White via a text exchange several months ago that he was tired of being ignored when it came to talk of the cream of the fighting crop.
"I told Dana I watch all these interviews and all these people talking and no one says my name when they're talk about the greatest fighters anymore. And I really don't like that, it really bothers me," Penn told a media conference call in advance of his showdown with MacDonald in Seattle.
"And I know it's my fault. I know I'm the reason why people don't talk about me when they talk about GSP (Georges St-Pierre) or Anderson Silva. My name was always in the mix. It's never in the mix any more.
"And I told Dana I've got a real problem with that. And that was actually a big part of my motivation to come back and be strong and do a good fight here on Dec. 8."
Benson Henderson defends his lightweight title against Nate Diaz, Nick's younger brother, in the main event at the Key Arena.
Penn has had plenty of time to prepare. Originally slated for September at Toronto, the bout was delayed when MacDonald suffered a cut in training that needed 38 stitches to prepare.
"I started this training camp at probably 40 per cent body fat," Penn said. "I was just going to get into shape a little, go there and give it my best effort.
"But as things started going on, Rory ended up pulling out 10 weeks before the last fight and he was saying that he saw me, I looked fat and a bunch of different things, that he'll probably end up killing me in the ring.
"That really lit a fire under my butt and I think I'm down under 10 per cent (body fat). I'm ready to go. I'm expecting the best B.J. Penn that I've ever seen. We'll just see how this all plays out."
Penn said he never lost his fire to fight, although he might have lost the desire "to prepare properly."
Not this time.
"We took my belly off," Penn said of his extended training camp.
The Hawaiian said he had got the itch to fight again, saying it was "better than staying home, sitting on the couch."
"I realize I can't do this for ever, so I might as well make the most of it I can," he said.
"As far as fighting with Rory, I think he's a great opponent, he's an up-and-comer, he's one of the top guys," he added. "Everyone says he going to be a champion soon."
Penn said fighting MacDonald also gave him another crack at Montreal's Tristar Gym, where St-Pierre, the current welterweight champion and MacDonald both train.
"They're a great team," he said of Tristar.
Just 23, MacDonald is considered by some as the heir apparent to GSP, who has two wins over Penn.
The native of Kelowna, B.C., who moved to Montreal to train at Tristar, is 4-1 in the UFC and has won three straight in impressive fashion since losing to Carlos (Natural Born Killer) Condit — beaten by GSP at UFC 154.
"There's a lot of hype behind him," White said of MacDonald. "A lot of people that I respect in the business say good things about him.
"I think this is a big fight for Rory. The one thing that you always know about B.J. Penn, you're not going in there and running over B.J.
"B.J.'s going to fight until the end. He doesn't cut, he's never been knocked out. B.J.'s a tough guy, man, and B.J.s a crafty guy. He's got good standup, obviously great submissions. His wrestling is good.
"I think B.J. is going to be a big test for him."
Penn is 1-3-1 since the beginning of 2010. That run includes losing his 155-pound title and a rematch to Frankie (The Answer) Edgar before beating veteran Matt Hughes, drawing Jon Fitch and losing to Nick Diaz, all three at 170 pounds.
MacDonald, then 20 by a few months, became the youngest fighter on the UFC's books when he signed a four-bout deal in the fall of 2009. Five fights later, he remains the seventh-youngest.
Penn and MacDonald have snapped at each other over Twitter, with Penn — no stranger to trying to get under people's skin — making some thinly veiled drug references about MacDonald.
The Canadian agreed to undergo random drug testing. He says he has already passed two tests.
Asked if he thought MacDonald was cheating, Penn said: "I'm sure there are people in all sports that bend the rules but I'm not going to sit here and point the finger."
"If I was going to make a comeback I want to make it as safe as I can for when I step in the ring," he added. "And that's all that this was about. I'm not pointing the finger, I'm not saying 'Rory, you do steroids' or anything like that. It has nothing to do with anything like that. I'm just protecting my own butt."
Penn says fighting is more than a sport for him.
"This isn't putting a ball in a hole, this has always been a fight for me," he said.
And he says he welcomes trash-talk from opponents, saying it fires him up.
"It's just all wonderful. I couldn't ask for anything more."
NOTES — UFC president Dana White says Georges St-Pierre, heralded as the UFC's top pay-per-view draw, did not disappoint in his recent return from a lengthy injury layoff at UFC 154 in Montreal. "The king of pay-per-view is back, let's put it that way," he said.