NEW YORK -- Mets catcher Mike Piazza will undergo a medical test Friday to determine the cause of the tendinitis in his left wrist.

Piazza, who will have an MRI, has not started the past three games because of the injury.

"If the picture is fine, it's just tendinitis,'' Piazza said. "Then I'll take a day or two and try to finish the year the best I can.''

The Mets don't think the injury is anything more serious than tendinitis.

"We're just trying to confirm the negative,'' general manager Steve Phillips said. "I don't anticipate it's a DL thing.''

Vance Wilson started in place of Piazza, who is hitting .285 with 23 home runs and 73 RBI.

Piazza grounded out as a pinch hitter in the eighth inning of New York's 5-3 loss to San Diego on Thursday.

Piazza, who will undergo an MRI, originally hurt the wrist Aug. 1 tagging out Houston's Richard Hidalgo at home plate. It began to bother him before batting practice Tuesday. He took a few swings, decided not to hit, and was scratched from the lineup shortly before the start of the game.

"They can't figure out what is causing it,'' manager Bobby Valentine said.

Piazza hit his 327th home run as a catcher Aug. 6, tying Johnny Bench for second place on the career list. Carlton Fisk hit 351.

The Mets have lost five straight games, dropping a season-worst four games below .500.