Contreras was examined by team doctors in Philadelphia on Monday and will undergo an MRI on Tuesday, general manager Ruben Amaro Jr. said.
"He had milder symptoms today than he had yesterday, which was nice," Amaro said. "He'll have some diagnostic work done tomorrow and we'll have more for you tomorrow."
Asked also if the injury might have come from overuse, Amaro responded: "No. Not at all. Guys get sore. It's part of the game of baseball and part of sports."
With regular closer Brad Lidge on the disabled list indefinitely because of a sore right shoulder, Contreras was moved into that role. Contreras has pitched in eight games, saving five in his five opportunities. He pitched five times in seven games from April 15-21, including three in a row, although that trio of games was broken up by a rainout.
His last appearance was Thursday night in a game he saved at San Diego. Right-hander Ryan Madson saved the games on Friday and Saturday, and with Contreras down, Antonio Bastardo came on in the ninth to save the game on Sunday after Roy Halladay threw 130 pitches.
There were those in Philadelphia who questioned Manuel's decision to let Halladay throw that many pitches this early in the season and those who thought the Phillies had burned out Contreras. Manuel wouldn't give any specifics about who he was directing his comments to on Monday. He just said he hears it from the media, "from everybody around the ballpark."
"I didn't think he's been used a lot," Manuel reiterated about Contreras. "What's today, April 25? I already hear our guys need days off, our guys need this, need that. I've been hearing that for four or five years now. The object every day we come to the ballpark is for us to win the game. Now do you want to win or do you want to lose? To me, I want to win, so I'm going to put the best team I can on the field that day. If I'm accountable, let me do my job.
"I get tired of hearing people say, 'We want him healthy in October.' October is a long way off. It's April 25. A lot of people are not going to make it to October."
But all that criticism is the nature of the game and goes with the territory, especially in Philadelphia. Isn't it?
"That doesn't mean you have to like the nature of the game," Manuel said. "I don't think I look like a nature boy."