Former No. 1 starter Ricky Romero will be recalled from Class A Dunedin to start against the Mariners while Johnson has been sent to Florida where he will undergo further tests.
"He'd been feeling pretty good," manager John Gibbons said. "He hadn't thrown for a while, then he played catch on the side and he felt pretty good but he still wasn't airing it out. So he was airing it out a little bit before he stepped on the mound [for a bullpen session]. That's when [there were] alarm bells."
The injury is another setback for Johnson, who has been forced to deal with a series of ailments during his career. The 29-year-old had a right-shoulder injury in parts of both the 2010-11 campaigns, while in '08 he underwent surgery on his right elbow.
Johnson, who is 0-1 with a 6.86 ERA in four starts, received a clean bill of health in New York last weekend when an MRI didn't reveal any structural damage to his elbow. The only problem found was inflammation in his right triceps muscle, but he will now receive a second opinion.
He hopes the second diagnosis will be the same as the first and he will only have to miss a couple of additional starts. The DL stint has been backdated to Monday so the club could promote right-hander Brad Lincoln from Triple-A Buffalo.
Lincoln had been previously called up on April 25 before being sent back down Sunday. If Johnson's DL stint was backdated prior to Monday, then Lincoln would not have been eligible to return.
"He'll get checked out down there and make sure nothing's really wrong," Gibbons said. "He had the MRI and it came out fine, but something's still bothering him."
Johnson's injury means Romero is set to make his return a lot sooner than originally expected. Romero was optioned to the Minors after a frustrating Spring Training so he could continue working on his altered mechanics.
The Blue Jays have been looking for Romero to pitch with a more direct line to the plate. The club has changed his delivery so that Romero no longer brings his hands above his head, and the hope with all of the alterations is that he'll stop throwing across his body as much as he did before.
The decision to bring Romero back to the Major Leagues comes just one day after Gibbons said the club wanted to give him at least one more start at Dunedin. The Johnson injury changed that plan, but Toronto's manager still feels as though Romero is ready to put the new mechanics into action.
"If we didn't feel good about it, and he didn't feel good about it, he wouldn't be coming here," Gibbons said. "He's been around the league a while. He's had a lot of success here. It's not like he's battling that, being a greenhorn."
Romero spent most of April pitching in simulated games at extended spring training. He made just one official appearance for Dunedin on April 27 and allowed just one run over seven strong innings of work while hitting 93 mph on the radar gun.
Perhaps the most encouraging thing about Romero's debut, though, was that he didn't walk a batter. The lack of command was his biggest obstacle this spring, and also last year, when he walked a career-high 105 batters in 181 innings.
"Everything we've seen, it's been really good," Gibbons said. "Of course, he pitched that one game down there in Dunedin and looked really good, but [roving pitching instructor] Dane [Johnson has] been watching him from the get-go. He's had a lot of work, he just hasn't had a lot of game action."
The Blue Jays announced after Thursday night's 3-1 loss to the Sox that right-hander Justin Germano was designated for assignment to make room for Romero on the 25-man roster. Germano made one appearance for the Blue Jays this season and surrendered a pair of runs in two innings of work.