HOUSTON -- Scott Feldman led the American League with a 1.33 ERA in August.
The start of September didn't go nearly as well for the Houston starter, who left in the third inning of Tuesday night's 7-5 loss to the Seattle Mariners with discomfort in his right shoulder.
He walked five and gave up three hits and three runs in 2⅔ innings before his early exit.
Feldman walked two straight batters before grimacing after a pitch to Mark Trumbo that resulted in a fly out for the second out of the third inning. Trainers visited with him briefly on the mound before he left the game and was replaced by Vincent Velasquez.
"It felt worse and worse," he said. "It felt tight at the beginning of the game, but [then] it started feeling to where it was actually starting to hurt. You got to try to know the difference between tightness and putting yourself and the team in a bad position. Once I started hurting, I don't want to go out there and tear my shoulder."
He'll have an MRI on Wednesday.
"It was just one of those nights where it was going to be a battle," Feldman said. "I didn't have my best stuff or command or anything. I was trying to battle through it. Then, unfortunately, the shoulder started hurting."
He returned from the disabled list on July 18 after missing almost two months following arthroscopic surgery on his right knee.
Information from The Associated Press was used in this report.