MacDonald's fight against former lightweight and welterweight champion B.J. Penn is currently listed as the co-main event for the yet to be fleshed out Sept. 22 show at the Air Canada Centre.
Penn, 33, retired in the cage after losing a unanimous decision to Nick Diaz at UFC 137 last October.
But the former lightweight and welterweight champion reconsidered and the UFC approached MacDonald, saying the Hawaiian star was interested in fighting him.
MacDonald (13-1) was surprised, but intrigued.
"I took it to the next step and made it public, asking B.J. if he'd like to fight me," MacDonald said Monday. "He said yes and now we're putting it together.
"And now that the Junior (heavyweight champion Junior Dos Santos) and Cain Velasquez fight is apparently not happening, I think it's a good opportunity for me to step in and hopefully the UFC will make us the main event here in Toronto. ... I have a lot of fans and I think I'm ready to sell the fight."
Asked how he sees a fight against Penn unfolding, MacDonald replied: "Me winning and stopping him before the distance."
Just 22, MacDonald is the fourth-youngest fighter in the UFC (behind Max Holloway, Michael McDonald and Charles Oliveira).
But he has ice water in his veins and has turned heads with his rapid progression in the cage. He dismantled Nate Diaz, Mike Pyle and Che Mills in his last three UFC outings.
MacDonald, a native of Kelowna, B.C, who trains in Montreal, likes the idea of facing Penn.
"He was one of my favourite fighters growing up," he said. "So if I could get that fight before he retires, that's amazing for me. I'm really excited."
Tom Wright, the UFC's director of Canadian operations, likes the old-school-new school matchup. He called talk of making it the main event premature but said it could happen.
"But I can tell you that if it was the main event ... a five-round (fight) between those two would be fascinating to watch."
MacDonald had wanted to fight Carlos Condit, who handed him the only loss of his career at UFC 115 in June 2010. The Canadian led for most of the fight, before a Condit rally late in the third round resulted in a TKO.
But with champion Georges St-Pierre recovering from knee surgery, Condit defeated Nick Diaz for the interim title and is currently waiting for GSP to return to action so they can decide who is the real champion.
That left MacDonald out in the cold.
"I feel like I'm ready to fight Condit but unfortunately he has to fight Georges first," he said.
"It's a good fight for me right now," he added of facing Penn. "Why not?"
He called Penn "a born fighter."
"He's ready to fight, whatever weight class it is," he added. "I have the same approach so I respect him at whatever weight he wants to fight."
Penn (16-8-2) has said the MacDonald fight interests him because there is a lot of hype around the young Canadian and because MacDonald is part of the same Tristar Gym camp as St-Pierre, who beat him twice.
MacDonald, however, says the GSP connections are not part of his thinking.
"I'm just representing myself. Tristar's my gym and they help me train. I fight for me."
Whatever happens in the welterweight ranks, MacDonald said he won't fight the 31-year-old St-Pierre (22-2).
"It would put a friendship on hold," he said. "It would put my situation at Tristar in danger. And I want to make that my home for a long time."
MacDonald also says he is a couple of years away from reaching his peak.
"When I'm 25, we'll see — see where we are at that point, if Georges is retired or where he is. We don't know"
UFC 152 tickets go on sale to the general public July 27.