NEW YORK (AP) — Struggling pitcher Oliver Perez was placed on the 15-day disabled list Saturday by the New York Mets because of a sudden — and somewhat convenient — injury to his right knee.

The move was made retroactive to Tuesday, and it solved a roster problem for the Mets. Left-hander Jonathon Niese came off the disabled list to start Saturday against Florida, so New York needed to open a spot for him.

By putting Perez on the 15-day DL, the Mets avoided having to send a pitcher to the minors.

In the second year of a $36 million, three-year contract, Perez lost his spot in the rotation this season and was banished to a mop-up role in the bullpen. The team had hoped he would accept a minor league assignment to work on his troubles, but he had the right to refuse.

Perez arrived Friday at Citi Field and complained of discomfort in his right knee, the team said. The left-hander, who had surgery on the same knee in September 2009, was examined by team physician Dr. Struan Coleman and had an MRI. He was diagnosed with patella tendinitis, according to the club.

New York manager Jerry Manuel said Perez told him Friday that his knee started bothering him after his most recent outing, Monday in San Diego.

"He never felt any discomfort in his knee, other than when I came in yesterday," Manuel said.

The manager was asked if he thought it was possible Perez was pitching hurt all season.

"I thought that with the velocity not ever getting to what I saw in 2008, that always concerns me to some degree," he said. "But the athlete tells you that he's fine, he's fine, doesn't feel anything, you have to give him the benefit of the doubt."

Perez will begin his rehabilitation at the Mets' complex in Florida and it is not known when he will begin throwing again. If the club wants to send him out on a minor league rehab assignment at some point, it would need Perez's consent to do so.

"He says he's not able to pitch the way it is right now," assistant general manager John Ricco said. "When a player tells you he's injured and a doctor confirms that, from where I sit, that's what the DL is for."

The 28-year-old Perez is 0-3 with a 6.28 ERA in 11 appearances, including seven starts. He has allowed 43 hits over 38 2-3 innings with 30 strikeouts and a team-high 33 walks.