UFC 129, which will be headlined by Georges St-Pierre vs. Jake Shields for the UFC welterweight title, will take place at the Rogers Centre on April 30. The card will be the organization's first in Toronto.
Diaz (13-6) is coming off a unanimous decision loss to Dong Hyun Kim at UFC 125 earlier this month. That defeat marked his first since moving up to 170 pounds. Prior to the loss, the 25-year-old Diaz went 2-0 as a welterweight.
The 21-year-old MacDonald (10-1) hasn't been seen since he lost his first professional fight to Carlos Condit at UFC 115 last June. He was supposed to compete at UFC 123 against Brian Foster, but an injury forced him to pull out of the fight. Since the Condit loss, he has moved to Montreal to train with St-Pierre and the rest of the team at the Tristar Gym.
Other fights scheduled for UFC 129 include Randy Couture vs. Lyoto Machida and Phil Davis vs. Matt Hamill.