Mariano Rivera sustained a torn anterior cruciate ligament and mensicus in his right knee while shagging balls during batting practice Thursday evening.
The Yankees' 12-time all-star pitcher caught his cleat where the grass meets the warning track in Kansas City, Mo., and fell to the dirt.
Rivera was given an MRI exam during the Yankees' 4-3 loss to the Royals and it revealed the injury, which will likely end his season.
"It's not a good situation, but again, we've been through this before, and we're being tested one more time," Rivera told reporters. "It's more mentally than physical, you know? You feel like you let your team down."