Mike Piazza was placed on the 15-day disabled list yesterday with a broken bone in his left hand, leaving the Mets without another key player as they try to make a push for the playoffs.

The move was made retroactive to Wednesday. To replace Piazza on the roster, the Mets purchased catcher Mike DiFelice’s contract from Triple-A Norfolk.

Piazza was hit by a foul tip at the base of his glove hand, below the thumb, on Tuesday night against Pittsburgh. He caught one more inning before leaving that game and has not played since.

“You get hit in a spot like that, there’s nothing you can do. It’s just been really painful,” he said. “It’s just bad luck.”

Piazza’s hand was still sore and swollen before yesterday’s game against Washington, and he sounded disappointed about having to go on the DL.

“It’s just obvious that the progress wasn’t made to push it any longer, with the roster situation,” he said. “Obviously, I’m frustrated, but the key is obviously for this ballclub to keep rolling and for myself to get as healthy as I can to help this team.”

Playing perhaps his final season with the Mets, Piazza will be eligible to return on September 1 – but he might not be ready by then. And when he does come back, it could take time to regain his stroke. The 12-time All-Star is batting .259 with 14 homers and 55 RBI. He plans to travel with the team on its upcoming trip.

“The only thing that’s bad about the injury is you’ve got to get your timing back. He can’t swing a bat,” manager Willie Randolph said.

Backup catcher Ramon Castro has played well in Piazza’s absence, and will continue to see regular action. DiFelice probably will get a start or two on the trip, Randolph said. DiFelice was with the Mets earlier this season, going 0-for-8 while Castro was hurt.