It's a scary time for the New York Yankees right now.

Mariano Rivera, the all time saves leader, had to be carted off the field before Thursday's game with the Kansas City Royals after injuring his knee during pre-game batting practice.

Rivera generally shags fly balls before the game as a way of staying in shape, and was hurt when he leapt for a ball.  He grimaced and had to be taken off the field on a cart.

An MRI is being performed.