David Robertson's tenure as Mariano Rivera's replacement is on hold.

The new Yankees closer showed up at Yankee Stadium Monday morning for the home opener complaining of a sore groin, and Joe Girardi said an MRI revealed a Grade 1 strain. That diagnosis landed Robertson on the disabled list, and suddenly the back end of the Yankees' bullpen is in flux.

Shawn Kelley threw a perfect ninth inning in the 4-2 victory over the Orioles for his first career save. Girardi said he likely will continue to go to Kelley for saves in Robertson's absence.

"It's not what you want, but you've got to figure it out,'' Girardi said after the game. "Other guys will get a chance to step up, and you expect them to step up and get the job done in his absence.''

Rivera, who retired last year after a stellar 19-season career, took part in the ceremonial first-pitch festivities along with Andy Pettitte, Jorge Posada and Derek Jeter.

Rivera's first pitch took place roughly around the time the Yankees learned the severity of Robertson's injury, not the way the team had hoped to pass the baton between the relievers.

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"I would have loved to have been out there pitching the ninth,'' Robertson said, "especially on Opening Day here.''

Robertson doesn't know when he hurt himself, only that he started feeling his groin muscle "aggravating me'' about five or six pitches into Sunday's outing in Toronto. The grabbing sensation got worse with each pitch, but he didn't report it to Girardi or the trainers at the time, saying his focus was on doing his job and finishing out the victory, which he did.

But when he woke up Monday, he said the soreness had gotten worse. He knew then he wasn't going to be able to pitch in the home opener.

Robertson said he doesn't expect to miss more than 15 days but said he's been given no timetable. "It's definitely a little frustrating,'' he said. "It seems like something always happens to me. But there's nothing I can do about it. There's no way I can go out there and be 100 percent and do what I do the way I feel right now.''

With Robertson out, the Yankees can only hope their bullpen performs as well as it did Monday. Matt Thornton and David Phelps combined to get the final two outs of the seventh inning with the potential tying runs on base, and Adam Warren struck out two in a scoreless eighth.

Kelley admitted to having butterflies when he walked out of the bullpen for the ninth, and he joked that he had a hard time remembering what happened in the inning. But whatever nerves he experienced, they didn't show in his nine-pitch, 1-2-3 inning.

"Opportunity presents itself all the time,'' Kelley said. "I would give anything for David to be out there with us. I know he's upset. But like I said, we need to move forward.''