Robertson sustained the injury during his appearance against the Toronto Blue Jays on Sunday.
In the Yankees first season without Mariano Rivera since the late nineties, Robertson was primed to be the heir.
He still is, of course, but his absence could be an opportunity for another pitcher to claim the role in New York -- if only for a brief period.
Robertson, 28, was exceptional last season in the set up role, pitching to a 2.04 ERA with a 1.04 WHIP in just over 66 innings. Unless he has a mental breakdown in the new role, he should be a valuable weapon for the Yanks as they attempt to lock up wins late in games.
While Robertson is shelved, left hander Matt Thornton and righty Shawn Kelley could serve as the interim closers if the Yankees look to play the match ups. The younger guys in the pen -- Dellin Betances, Adam Warren, David Phelps, and Vidal Nuno could get a shot as well, but when Robertson is healthy, he's all but certain to return as the closer.