Sheets had to leave the game due to tightness in his left hamstring at the start of the bottom of the second inning. Club officials said he would be evaluated on Wednesday.
"It didn't pop," Sheets said. "I felt it when I went out there, and it just gradually got worse and worse.
"I felt great in the first inning. I thought I was going to get in a nice groove tonight. Then something happened. I don't know. I guess we'll see tomorrow."
Sheets, the Brewers winningest pitcher with a 12-5 mark, went out to the mound to warm up for the second inning, but called trainers out before facing the first batter. After being examined briefly, he tried a short jog off the mound and back before the decision was made to call it a night.
Sheets left trailing 1-0 after the Astros jumped on him for two hits in the first inning. He threw only 12 pitches, nine of them strikes.
What concerned Sheets was it hadn't gone away 30 minutes after the game ended.
"I feel it a little bit still," he said frowning. "That's what makes me not know if it's just a cramp or not."
Manager Ned Yost was remaining optimistic.
"It's not even a strain," Yost said after the game. "It's just a cramp.
"When it got tight and started cramping, we just decided not to push it. We'll see tomorrow."
It was Sheets' shortest outing of the season. He left a game after just three innings against the Cubs on April 25 with a strained right groin, but didn't miss a start after that game.
He then lasted only three innings against Colorado on July 14 when he sprained his right middle finger. He went on the 15-day disabled list and missed 40 games after that.
"I'm just mentally fried from injuries this year," Sheets said. "The guys really picked me up by coming back and winning tonight. But I don't understand what's happening with me and injuries this year.
"It took me down pretty well tonight."
Jim Carley is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.