"That was a scary moment," Mets manager Bobby Valentine said.
Piazza, who struck out in the first, was examined by assistant trainer Scott Lawrenson behind home plate while Valentine stood waiting nearby. He attempted to grip a ball before making his way off the field.
"It just kind of went numb," Piazza said. "It might have hit in a sensitive spot. Bobby asked me if I wanted to throw the ball and I told him I couldn't really feel the ball."
Piazza had a black sleeve on his thumb to keep the swelling down as he talked to reporters following the game.
"I'm going to treat it throughout the night and tomorrow morning, and see how it feels," Piazza said. "If it feels better, then it might be good enough to play. If not, I don't see it going past maybe one more day."
Armando Benitez pitched the ninth for his 42nd save, topping the team record for saves he set last season.
The Mets remained four games behind division-leading Atlanta, which won, 4-1, in Florida. New York has won seven of its last eight games, and 24 of 30.
Rusch (8-10) allowed one run in the first and held Montreal scoreless before leaving with two outs in the eighth after Orlando Cabrera's double put runners at second and third.
In other NL games:
* At Houston, Mark McGwire broke out of a slump with two home runs and Darryl Kile allowed four hits in seven shutout innings, leading St. Louis over the Astros, 5-1. The victory pulled the Cardinals within 3 1/2 games of Houston in the NL Central Division.
Before hitting his first homer, McGwire was 1-for-19 with 10 strikeouts in his previous 20 at-bats, including strikeouts in his first two at-bats of the game.
* At Los Angeles, John Vander Wal drove in two runs in the ninth as San Francisco topped the Dodgers, 6-4.
* At Pittsburgh, Ricky Gutierrez had three hits and drove in two runs and Chicago ended a four-game losing streak with an 8-4 win.