In this image taken from video, New York Yankees' Mariano Rivera, center, grimaces after twisting his right knee shagging fly balls during batting practice before a baseball game with the Kansas City Royals, Thursday, May 3, 2012, in Kansas City, Mo. The Yankees closer was later carted off the field and sent for further tests. (AP Photo/YES Network) MANDATORY CREDIT

Associated Press
In this image taken from video, New York Yankees' Mariano Rivera, center, grimaces after twisting his right knee shagging fly balls during batting practice before a baseball game with the Kansas City Royals, Thursday, May 3, 2012, in Kansas City, Mo. The Yankees closer was later carted off the field and sent for further tests. (AP Photo/YES Network) MANDATORY CREDIT
In this image taken from video, New York Yankees' Mariano Rivera, center, grimaces after twisting his right knee shagging fly balls during batting practice before a baseball game with the Kansas City Royals, Thursday, May 3, 2012, in Kansas City, Mo. The Yankees closer was later carted off the field and sent for further tests. (AP Photo/YES Network) MANDATORY CREDIT
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