The 28-year-old received an MRI on Thursday, which revealed the issue. The flexor tendon is the major attachment for the muscles in the forearm and wrist that powers the wrist and fingers.
• Spring Training: Tickets | Schedule | More
"Last Friday in live BP, I felt it tighten up on me a little bit," Loup said. "I took five days off, tested it out yesterday, felt a little better, but still felt it. So we went ahead and got an MRI. It showed that I had a flexor strain in my left elbow, so they told me to take another two weeks off, not throw and then go from there."
The timing of Loup's injury likely means he won't be ready for the start of the regular season since he will not throw again until at least March 18, meaning a stint on the disabled list seems all but guaranteed.
Loup typically likes to throw 10-12 innings during Spring Training, and he estimated that he could probably get away with just five to six innings of preparation. That likely won't be able to happen prior to the season opener on April 3, because he will need to gradually build up in bullpen sessions before taking the mound in a Grapefruit League game.
Loup has been one of the more durable relievers on the roster since he broke into the league in 2012. He tossed at least 68 innings from 2013-14, and he had a high workload again last season with stints in Toronto and Triple-A Buffalo. Blue Jays manager John Gibbons conceded that the high number of innings possibly caught up with his versatile reliever.
"There's no tear in there, but it's still pretty sore," Gibbons said. "If you look back the three years I've been here, he has probably been our most durable guy. He was the one guy, when a lot of guys were beat up, he was still available and would always take the ball. Maybe a little bit caught up with him. Hopefully it's a minor thing -- apparently it is -- but we've seen things where it can drag on pretty good, too."
The injury opens the door for switch-pitcher Pat Venditte to crack the 25-man roster. Venditte throws with both hands, but the club particularly likes him from the left side, where he limited hitters to a .116 average and a .447 OPS in 47 plate appearances in 2015.
Other candidates for the second left-hander spot include Chad Girodo and Wade LeBlanc, and to a lesser extent, veteran Scott Diamond, who is expected to begin the year in the rotation for Triple-A Buffalo.
"You can throw most of the lefties in camp that do that job [into the mix]," Gibbons said. "You never really know. Everybody in baseball, forever, is always looking for left-handers who can get lefties out. I think those guys are more rare these days. You see a number of your left-handers that are better against righties because they throw the changeup.
"You're always looking for those guys who can be really tough on lefties. Really, most of your top hitters in the game are your left-handed hitters. We'll give them all a look."