DETROIT -- Left-hander Patrick Sandoval, who has enjoyed a breakout season in the Angels' rotation this year, was diagnosed with a lower back injury that could force him to miss the rest of the season, manager Joe Maddon said Wednesday.

Sandoval was officially diagnosed with a left lumbar spine stress reaction, and there’s no timetable for his return after being placed on the 10-day injured list. He’s posted a 3.62 ERA with 94 strikeouts in 87 innings, including a 3.39 ERA in 14 starts after an initial stint in the bullpen. He was scheduled to make his 15th start of the year on Thursday against the Tigers, but left-hander José Quintana will start in his place.

“It’s a lower back issue that started bothering him after his last start,” Maddon said. “And because of that, some testing was done and we have to shut him down for now. That’s what I know. It really came out of nowhere for us. It’s an unusual injury, so we’re still trying to collect information.”

Sandoval's velocity was down in his start against the Astros on Friday, with his fastball 2.2 mph slower than his season average. But he said after his start that nothing was bothering him physically.

Even if Sandoval misses the rest of the year, the 2021 season was still a personal success, and he set himself up to be a big part of the rotation for next year. Sandoval, 24, remains under team control through 2026.

Quintana will now rejoin the rotation after being moved to the bullpen in June. He posted a 3.86 ERA with 20 strikeouts and three walks in 16 1/3 innings as a reliever after posting a 7.22 ERA with 50 strikeouts and 26 walks in 33 2/3 innings as a starter.

“'Q' has done everything we’ve asked him to do,” Maddon said. “We've given him some opportunities out of the bullpen. For the most part, he’s done really well. This guy’s workout routine is among the best I’ve ever seen, so he’s physically ready to do it. It’s something he wanted to do it again. So all those factors are in place. He’s earned this opportunity.”