DETROIT -- The Tigers have lost a third starting pitcher to the injured list. This time, it’s right-hander Jordan Zimmermann, who was placed on the 10-day IL on Friday with what the team is classifying as a sprain of the ulnar collateral ligament in his right elbow.

For now, it does not appear to be a season-ending injury.

Zimmermann left Thursday’s 7-3 loss at Boston after three innings with elbow discomfort, having allowed five earned runs on five hits with three walks. He underwent an MRI exam on Friday morning in Chicago, which was reviewed by team physician Dr. Stephen Lemos.

“It did show that he has a sprain to his UCL ligament,” Tigers head athletic trainer Doug Teter said. “He does have some damage to the ligament itself, but in Dr. Lemos’ opinion, it’s not to the point where you need to pull back and do surgery or anything.”

Zimmermann will wait five to seven days to let his elbow calm down and the inflammation subside before beginning what Teter called a conservative rehab program. From there, they’ll go day by day based on how Zimmermann’s arm feels.

“Best-case scenario, we’re in that 3-to-4 week range right now,” Teter said. “That’s where we would like to be. But that being said, if he needs more time, we’re going to take the time we need to get this done right.”

If Zimmermann’s elbow doesn’t respond to rehab, surgery could be an option. But at this point, he has no plans to consult with any other doctors.

The injury is the latest setback for Zimmermann, who seemed to have the foundation for a rebound season before struggling his last four outings. The 32-year-old came to Spring Training with no health issues, having put offseason core muscle surgery and past back and neck problems behind him, and used camp to refine his pitches for the first time in a couple years. The work showed in the first two starts, during which he allowed one run on seven hits over 13 2/3 innings against the Blue Jays and Yankees.

“Obviously I’m frustrated,” Zimmermann said. “I had a really good spring and I felt good coming out the first couple starts. You want to be out there with these guys, battling every five days, and to not be able to do that for a few weeks is tough. But at the end of the day, it could’ve been a lot worse.”

The results turned worse over the last four starts, with 19 earned runs allowed on 23 hits over 16 2/3 innings, including six home runs. Opponents hit Zimmermann for a 1.124 OPS in that span.

Asked whether the elbow was a factor before his last start, Zimmermann said, “That could’ve had something to do with it. I’m not going to say it was or wasn’t. Obviously, I have to go out there and do my job and make my pitches, and I wasn’t able to do that the last couple starts. It just kept getting worse yesterday, and I had to get it looked at. The best-case scenario is what we have.”

Zimmermann had Tommy John surgery in 2009, but he hadn’t had any issues with his elbow in recent years.

To take Zimmermann’s place on the 25-man roster, the Tigers recalled right-handed reliever Zac Reininger from Triple-A Toledo. Reininger pitched for the Tigers on Tuesday in Boston, having served as the 26th man for the nightcap of the day-night doubleheader before returning to the Mud Hens.

How the Tigers fill Zimmermann’s rotation spot depends on the weather and the schedule. The team needs an extra starter for Saturday’s game against the White Sox, but the weather forecast calls for a mixture of rain and snow. If the game is postponed, the Tigers could theoretically go with a four-man rotation until May 11, thanks to scheduled off-days on Monday, Thursday and May 6.

If Saturday’s game is played, the Tigers are expected to recall Ryan Carpenter from Triple-A Toledo. The left-hander was scratched from his scheduled start for the Mud Hens on Thursday to make sure he’s ready just in case. He’s with the team in Chicago, but he isn’t on the active roster.

“It’s not looking good right now,” Carpenter said of the weather, “but you know how it goes. Could change.”

Zimmermann joins fellow Tigers starters Michael Fulmer and Matt Moore on the injured list. Fulmer and Moore are out for the season. Fulmer underwent Tommy John surgery near the end of Spring Training. Moore suffered a torn meniscus in his right knee earlier this month that required surgery.

“You can’t really predict injuries,” manager Ron Gardenhire said. “Pitching is a violent act.”

Mercer could be back next week

Jordy Mercer began his Minor League rehab assignment at Class A Advanced Lakeland on Thursday, going 0-for-3 with a walk and a strikeout. 

Mercer, who went on the 10-day IL a week and a half ago with a right quad strain, is scheduled to continue playing for Lakeland through the weekend. If all goes well, Gardenhire said Mercer could rejoin the team on Tuesday in Philadelphia.

Stewart rehabbing quad strain

While the Tigers were in Boston earlier this week, Christin Stewart was driving back and forth between Detroit and Ann Arbor to rehab his right quad strain. He’s back with the team this weekend, but just to do baseball activity. He was on the field on Friday afternoon working out with Tigers strength and conditioning coach Chris Walter, then Stewart took some batting practice.

“It’s a lot better than what it was,” Stewart of the injury.

The plan, according to Gardenhire, is for Stewart to continue working out with the Tigers, then head to Lakeland for a rehab assignment.