That's according to numerous reports out of Kansas City, where the Yankees closer slipped on the warning track as he was shagging fly balls before tonight's game against the Royals. According to CBSSports.com, the 42-year-old Rivera had to be carried to the cart by manager Joe Girardi and bullpen coach Mike Harkey.
This is from CBS Sports's C. Trent Rosecrans:
YES Network had video footage of Rivera shagging fly balls that showed a ball hit by Jayson Nix that went over Rivera's head and the all-time saves leader reached up to try to catch it, but it appeared his right knee buckled and he fell to the ground.
Rivera has shagged fly balls throughout his career in the big leagues.
The extent of the injury is not known.
Update (8:39 p.m.): Here's the video of Rivera's injury.