Mayza threw a pitch well behind Didi Gregorius and slowly went down to his knees holding his left forearm.
Ryan Tepera replaced the left-hander on the mound at Rogers Centre.
Mayza struck out 55 batters and posted a 4.67 FIP in 51.1 innings entering Friday, per FanGraphs. His 67 appearances are the most on the team.
A 12th-round pick in the 2013 draft, the 27-year-old is in his third season with the Blue Jays. He made his debut in 2017, when he started in Double-A and worked his way up to the majors.
In May 1999, Tampa Bay Devil Rays starting pitcher Tony Saunders blew out his arm in a seemingly similar fashion. His pitch sailed well to the left of the plate, and he immediately fell to the ground holding his left arm.
Saunders suffered a fracture to his humerus bone, which ended his season, and he never pitched in MLB again. He broke the bone again during a minor league rehab assignment in August 2000, forcing his retirement.
The Blue Jays have 14 more games left in the regular season.