BALTIMORE -- Orioles closer Zach Britton's season may be finished.
After a routine season-ending physical and MRI, the Orioles' left-hander will have a stem cell injection on his left knee Sept. 21.
Britton, who also had an MRI on the knee Aug. 24, said he injured it in Toronto in 2014 and has managed it since. He said he strained his MCL.
Manager Buck Showalter has said he's been aware of the knee issue, and others in the industry have apparently known about it, too. That may account for the tepid interest in Britton ahead of the July 31 non-waiver trading deadline.
Showalter said Britton would rest the knee for three-to-five days, but the closer said it was doubtful he'd pitch again this season after discussing it with team orthopedist Dr. Michael Jacobs.
"I would assume so based on my Intel from just talking to Dr. Jacobs, but you never say never," Britton said.
"I'm going to make myself available. I'm not shutting down by any means. 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 offseason, so I can be ready for spring training."
Britton didn't pitch during the Orioles' 9-0 loss to the Boston Red Sox before 16,908 at Oriole Park Sept. 20.
Showalter refused to concede Britton's season was over.
"We'll see how he is physically and where we are as a team," Showalter said.
At the trading deadline, most of the concern centered around Britton's left forearm, which sent him to the disabled list for most of the season's first half.
"The most important thing for me is to be healthy going into next season," Britton said. "That's the thing that they kind of preached to me from an organizational standpoint. Obviously, I've got the arm injury past me. Started to feel pretty good on the mound, managing the knee, which has been pitching through pain for two months, trying to help the team win."
Britton, who has a 2-1 record with a 2.89 ERA in 38 games, has 15 saves in 17 chances.
Britton's American League record streak of 60 consecutive saves converted ended Aug. 23, the day before he had the initial MRI.
Surgery for Britton isn't contemplated, and he feels the knee will manage itself with rest.
"Not anything that needs to be operated on, which is the good news," Britton said. "I thought maybe that was something that might have to be done in the offseason..
"I spoke with Dr. Jacobs through my exit MRI that I had today, something we were going to 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."
Left-hander Wade Miley, who allowed six runs while retiring just one batter in his Sept. 14 start, gave up six more in 4.1 innings against the Red Sox.
The Orioles played without third baseman Manny Machado, who was unavailable with flu-like symptoms, and shortstop Tim Beckham, who had an abscessed wisdom tooth removed.
Ryan Flaherty started at third base, his first start in the infield since May 18, the day before he began a three-month stay on the disabled list with a sore right shoulder. J.J. Hardy started at shortstop.
Boston left-hander Chris Sale struck out 13 Orioles and allowed just four hits in eight innings.
The Orioles fell to 73-80. At seven games below .500, they've matched their season-low while losing 12 of 14.
Left-hander Tanner Scott, who was added to the roster last weekend, made his major league debut in the eighth.
Scott gave up two runs on two hits, walked two and struck out one while showing off a 100 mph fastball.
NOTES: Left fielder Trey Mancini has a 10-game hitting streak. … Scott is the 50th player used by the Orioles this season. … RHPs Gabriel Ynoa (1-2, 4.18) and Matt Anderson (5-3, 4.44) are the scheduled starters when the Orioles and Tampa Bay Rays begin the first of a four-game series Sept. 21.