Sheets' injury was initially announced as tightness in his right forearm. But after Milwaukee's 6-2 win over the Chicago Cubs, Sheets said it was his elbow that forced him out and had been giving problems for some time.
"It hurts. It wasn't making me nervous. It was the lack of ability to make pitches," Sheets said.
The Brewers won without their All-Star ace, and stayed a half-game behind New York in the NL wild-card race.
Will he pitch again this season as the Brewers try to make the playoffs?
"It's been so up and down that I would think it's very possible," he said. "There's been really good days and really bad days. There's been days where it's hard to move it and it's stiff. Then the next day it's great. So we'll see."
Sheets said he began to feel some discomfort in the bottom of the first when he faced Derrek Lee, the Cubs' No. 3 hitter.
"I don't know what happened today. My elbow has been bothering me for a while now," he said. "I started feeling it tug and it was in my elbow."
Asked if it was his forearm or his elbow causing the discomfort, Sheets said: "They kind of go together. But it's definitely my elbow. We got an MRI all ready earlier and it's not anything structural."
Sheets said he got the MRI exam about a week and a half ago after initially feeling discomfort when he pitched against the Cardinals in St. Louis on Aug. 26. He said the elbow had been feeling better and responding to treatment, but Wednesday night "it kind of went backwards."
The Brewers were leading 3-1 when Sheets departed. He gave up a solo homers to Aramis Ramirez in the second inning. Mark DeFelice came in to pitch for the Brewers.
Sheets, who entered Wednesday night's game with a 13-8 record, has been slowed by injuries. He left a game earlier this month after five innings with a tight left groin.
In the last two years, Sheets has battled a groin problem, a torn finger tendon, shoulder tendinitis, a tight triceps and a sore chest muscle.