CINCINNATI -- Reds starting pitcher Brandon Williamson said Wednesday he has a full UCL tear that will require Tommy John surgery.

The left-hander exited Tuesday night's game against Atlanta in the second inning with a strained elbow. He doubled over for a brief moment near the mound and was replaced by Fernando Cruz.

"It kind of came out of nowhere," Williamson said, adding that he had two MRIs to diagnose the tear.

Williamson, a second-round pick in the 2019 amateur draft, hurt his shoulder in spring training. The 26-year-old made his first major league start of the season Sept. 1. It was the first of four starts this year.

"It was just baseball the first inning," Williamson said. "I would say I felt a little off, non-physically wise. As far as the mechanics, I was maybe altering a little bit. The second inning I came out after a long first inning, I felt like I had to put a little extra, to get warm and get going. A long first at-bat again. There were no red flags for me. I felt fine. I believe the third pitch (to Michael Harris II), I threw a changeup. I felt a little tweak. I tell everybody, if you're a pitcher, you feel stuff all the time. I didn't think too much of it. So, the next pitch I just felt it."

Williamson allowed three runs over 13 innings in three appearances before Tuesday. He allowed three runs in the first inning before the injury. He finished the season with no record and a 3.77 ERA.

"He is still young and athletic," Reds manager David Bell said. "Obviously, he won't be on the mound Opening Day. I don't have any real information on the timeline. I think as soon as we find out, I hope we have a realistic goal of having him back with real time left in the season next year."