NEW YORK - Brad Lidge's performance for most of this season has been a mystery, but the Phillies offered a potential explanation yesterday.

Lidge, 32, was placed on the 15-day disabled list, retroactive to June 7, with a sprained right knee. Two recent MRI exams have shown no structural damage, the Phillies said, but Lidge has pitched with inflammation for most of the season.

The closer is 0-3 with 13 saves, six blown saves and a 7.27 earned run average in 28 appearances. He blew consecutive saves Friday and Saturday in Los Angeles.

The team does not know how long Lidge will be out, and said that eighth-inning specialist Ryan Madson will assume the closer's role in Lidge's absence. J.C. Romero will likely assume most of Madson's duties.

"It is a big blow for us to lose our closer," said general manager Ruben Amaro Jr. "He didn't have the same stuff. His slider was not as sharp, he didn't have fastball velocity, he couldn't drive off his back leg."

Lidge was not at Citi Field for the start of the series with the New York Mets last night, and attempts to reach him were unsuccessful.

"The decision was not Brad's," manager Charlie Manuel said. "The decision was out of his hands."

Amaro, Manuel, pitching coach Rich Dubee, trainer Scott Sheridan, and assistant general manager Scott Proefrock met with Lidge at the team hotel in Manhattan yesterday, according to sources. That was when management told Lidge he was headed to the DL.

"He did not suggest it," Amaro said. "I don't know how on-board he was with it, because he doesn't want to make any excuses, but he understands. Our medical people and staff took the decision out of it for him."

Lidge will undergo unspecified tests, possibly another MRI, before the team sets a schedule for his return, Amaro said. The team elected not to sideline Lidge when the inflammation emerged in April, but felt that attempts to pitch through the issue had resulted in inconsistency.

"The player never complained to the point where we needed to make a change" until now, Amaro said. "Even though he didn't complain about it every day, we believe it affected his performance and mechanics."

Dubee said that if Lidge continued to pitch while injured, he could alter his mechanics in a way that could risk further injury. "If he kept going on like this, [he could hurt his] arm, could hurt his back," Dubee said.

Dubee said that he asked Lidge about the knee Friday and Saturday. "He told me it wasn't right and he was concerned about what's going on and why it hasn't improved," Dubee said. "Some days it felt pretty good and other days it felt pretty bad. He was honest enough to say it wasn't right."

Speaking on national television during Sunday's game, Dubee said that Lidge's issue was strictly one of confidence. "How much do you want people to know about an injury?" he said yesterday. "Do you want the opposition to know that he's not moving very well on his knee and stuff? Sometimes you don't tell the whole story. . . . I don't think confidence is as much a factor as being concerned that you're healthy. I think the knee distracted us quite a bit."

Lidge underwent knee surgery during spring training of 2008, but he went a perfect 41 for 41 on save opportunities during the season and 7 for 7 in the postseason.

Amaro said that Lidge would not lose his job as closer, no matter how well Madson performs. "Brad Lidge is our closer," Amaro said. "He is indeed. He's done nothing to lose that job."

Does the general manager feel a need to trade for a relief pitcher? "We're trying to upgrade our staff," he said. "If we're going to miss Brad for a couple of weeks . . . it may put more focus on a bullpen piece or it may not. If I had my druthers, I'd address" both the rotation and bullpen.