Orioles closer Zach Britton will receive a stem-cell injection in his balky left knee Thursday afternoon, and will be out at least three-to-five days, though it’s almost certain his 2017 season has ended with nine games remaining.

Britton, 29, said an MRI as part of the club’s annual exit physical showed a slight worsening of a MCL strain that has bothered him for a couple months and has been an off-and-on source of discomfort over the years.

Orioles team physician Dr. Michael Jacobs suggested that Britton get the procedure now and, with rest, should be fine to begin his regular workout program this offseason. Surgery has been ruled out at this point.

“Not anything that needs to be operated on, which is the good news. I thought maybe that was something that might have to be done in the offseason,” Britton said Thursday night. “I spoke with Dr. Jacobs through my exit MRI that I had today, (it’s) something we were gonna do anyway. And he just felt like it was in the team’s and my best interest to maybe try to get the recovery process going on this right now. So that’s what we’re going to do.”

Orioles manager Buck Showalter left open the possibility that Britton could return before the season ends Oct. 1.

“The plan is do (the stem-cell injection) now and then see where we are in three to five days on a lot of different fronts,” Showalter said. “We’ll see how he is physically and where we are as a team.”

Given that the Orioles are 5 ½ games out of the second AL Wild Card spot with nine games remaining, it seems highly unlikely Britton will pitch again this year.

“I would assume (that he is done this year) based on my intel from just talking to Dr. Jacobs, but you never say never. I’m going to make myself available. I’m not shutting down by any means,” Britton said. “I’m going to go out there and keep myself in pitching form as much as I can. We’re going to do what we can to start the recovery process and not let it linger for longer than it needs to in the off-season so I can be ready for spring training.”

Britton was baseball’s best reliever in 2016, converting all 47 of his save opportunities and posting a 0.54 ERA in 69 games. This year, however, has been a physical nightmare for the lefty sinkerballer who had a 2.89 ERA in 38 games and converted 15 of his 17 save chances.

A strained forearm cost him two weeks in April and then two more months starting in early May. He’s been dealing with the knee discomfort, he said, for roughly two months, but pitched through it.

“Really frustrating. The good thing is that they’ve been minor injuries, nothing that needs surgery or anything that’s going to keep me out for a long time,” he said. “Yeah, it’s been frustrating. Kind of get back to the drawing board this offseason, with approaching my workouts. I think that’s where it all stemmed from, last season the workouts I did. I can see what I did, what I didn’t do the prior years and kind of get back to that. That’s the approach I’m going to take is, it starts in the offseason with how I prepare my body for the season.”

Brad Brach, who leads the club with 17 saves in Britton’s absence, likely will assume closing duties for the remainder of the season.