Johnson, 29, has missed more than six weeks with a right-triceps injury. He went 0-1 with a 6.86 ERA and 1.88 WHIP in 19 2/3 innings across four starts before the injury. Toronto acquired him from the Marlins as part of last winter's 12-player blockbuster trade.
Blue Jays starters have a 5.56 ERA, the worst in the AL and second-worst in baseball overall. Fellow offseason imports Mark Buehrle (5.42 ERA) and R.A. Dickey (5.18 ERA) have been disappointing as well.
The Jays also activated outfielder Rajai Davis off the 15-day DL. He missed close to a month with a strained side and has hit .284/.326/.395 with eight stolen bases in 26 games this year.
The Jays placed right-hander Ramon Ortiz on the 15-day DL to clear a roster spot for Johnson. Ortiz, 40, walked off the field in tears on Sunday after apparently suffering a serious and potentially career-ending elbow injury.