Janssen had been dealing with some soreness in the forearm recently but was able to pitch.
Janssen will not be replaced by another pitcher, but the move will open up a spot for the return of infielder John McDonald from the disabled list, where he had been since May 28 with a right hamstring strain. Toronto had been carrying eight relievers.
"It wasn't getting any worse or better," Janssen said after Thursday's 4-3 loss to Baltimore. "Sometimes it was just a little more difficult to get loose. The other night, it kind of caught up with me a little bit. I let them know, and this was kind of the next step. I'll have an MRI tomorrow and take a couple of days off. Hopefully, it's not a long thing and I'll get back quick. This is new to me. It's just a little different."
"[Wednesday] was the first night that he came forward to the point of where he could feel a little bit more in his forearm," manager John Farrell said. "Prior to that, as we check with all of our relievers, he felt like he was able to pitch through it.
"There was no reduction in overall stuff or performance. And yet he felt it a little, but [he felt it] more [Wednesday] night warming up. He will report to Florida tomorrow. He will go through our normal protocol for any pitcher in this case, where there's an MRI just to rule out anything, but any medical tests in the training room have not revealed any ligament issues at this point."
Janssen is 2-0 with a 2.93 ERA in 28 games this season.