An MRI exam on Sunday confirmed the extent of the injury, Mets manager Bobby Valentine said.
Piazza was injured while crossing first base on a groundout in the fourth inning of New York's 6-2 victory over the Arizona Diamondbacks on Saturday night.
Valentine said Piazza might be able to pinch hit as early as Monday night's series opener at Houston.
"It's not serious," Piazza said at the ballpark after the MRI. "It's day-to-day. It's one of those things that obviously we can deal with."
Piazza said he didn't think he'll be out more than three days.
"It feels a lot better," he said. "I'll probably ice it the rest of the day, treat it. Obviously I have a night off tonight, and tomorrow it should be a lot better, hopefully in the next 24 hours."
Piazza was 3-for-8 with three RBIs as New York won the first two games of its three-game series in Arizona.
Valentine said it's possible the Mets will make a roster move to compensate for Piazza's absence.
"I don't know if it requires it," Valentine said Sunday. "We'll see how he looks today."
Piazza is sixth in the National League in batting at .342. He leads the Mets in home runs with 29 and RBIs with 88.
Todd Pratt took Piazza's place in Sunday's lineup against Arizona.
©2000 The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed