ANAHEIM -- Angels right-hander Keynan Middleton knew something was amiss after throwing a changeup to retire Eduardo Escobar on a groundout in the eighth inning of the Angels' game against the Twins on Sunday afternoon.

"When it left my hand, I knew something was wrong," Middleton said.

The radar gun, too, signaled trouble: Middleton's next two pitches were fastballs clocked at 88.2 mph and 92.7 mph, significantly below his 97-mph average in 2017. Catcher Martin Maldonado subsequently called for a trainer, and Middleton was forced to exit the game without completing the inning.

It'll be a while before he takes the mound for the Angels again. Middleton said Thursday that he will undergo Tommy John surgery to repair a torn ulnar collateral ligament in his right elbow. The procedure, which typically entails a 12-to-18-month recovery period, will cost Middleton the remainder of the season and possibly the 2019 campaign as well.

"It's really unfortunate the situation that I'm in," Middleton said. "Our team is competing this year, so it just sucks not being able to contribute to the rest of that season."

Middleton had logged a 2.04 ERA over 17 2/3 innings and emerged as the Angels' primary closer early this season before landing on the disabled list with right elbow inflammation on May 1. The 24-year-old sat out nine games and then felt well enough to return to action. He logged two scoreless outings before injuring his elbow further Sunday.

"It all happened like it happened," Middleton said. "I want to be out on the field as much as I can, and I was feeling well enough to throw, so I came back and that's what I did. Unfortunately, this is the result."

An MRI exam on Monday revealed the damaged UCL, though Middleton traveled to Cincinnati to seek out a second opinion before deciding to undergo surgery. He said he will consult the Angels' doctors before settling on an exact date for the operation.

Middleton is the sixth Angels pitcher to suffer a UCL injury since 2014, joining Tyler Skaggs , Garrett Richards , Andrew Heaney , Nick Tropeano and JC Ramirez. Only Richards was able to avoid undergoing Tommy John surgery by opting for stem-cell therapy.

"Unfortunately, it's a common injury in our sport now," Middleton said. "It happens all the time. I'll come back from it. I'll work hard and hopefully I'll come back even stronger from it. That's what I'm expecting of myself."

Worth noting

Reliever Blake Wood (right elbow impingement) made his second rehab appearance with Class A Advanced Inland Empire on Thursday, allowing one run on two hits over two-thirds of an inning while striking out one.