Robertson felt discomfort Sunday five or six pitches into the ninth inning of a 6-4 win over the Toronto Blue Jays, his second save in three outings this season.
"It's really disappointing," Robertson said Monday after New York beat Baltimore 4-2 in its home opener. "I would've loved to have been out there pitching the ninth today, especially on opening day here."
Instead, Robertson had an MRI that revealed a Grade 1 strain and setup man Shawn Kelley earned his first career save. Kelly pitched a perfect ninth that ended with him catching a toss from first baseman Kelly Johnson and keeping the game ball.
"Exciting," said Kelley, who learned Sunday night he might fill in for Robertson.
Robertson doesn't expect his absence to last more than the required 15 days.
"Anytime I sit for a handful of days I feel good as gold," Robertson said. "I think it will be rest a little while, play catch and get a bullpen and get right back in there."
Robertson made a smooth transition to closer after Rivera retired at the end of last season with a major league-record 652 saves. An All-Star setup man, Robertson was only eight for 18 in save chances during six seasons before taking over the role.
He's allowed one hit in three scoreless innings this year.
The Yankees bullpen was impressive without him Monday.
Adam Warren pitched the eighth and after a leadoff walk he retired three straight, striking out All-Stars Adam Jones and Chris Davis to end the inning.
"We're going to try to step up and fill that void for a brief period of time," Warren said.
Also, first baseman Mark Teixeira had an MRI that confirmed he had a Grade 1 right hamstring strain. Placed on the DL Saturday, Teixeira thinks he will only need the minimum time out. He said he will begin an exercise program Tuesday.
"I knew that it wasn't anything major when I did it," Teixeira said.