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."
The 42-year-old Rivera, who has played for 18 years in the the major leagues, said that he will decide after the season whether to retire.
"At this point, I don't know," he said of ending his career. "Going to have to face this first. It all depends on how the rehab is going to happen, and from there, we'll see."
Rivera has saved 608 games and has five World Series rings.
After the Yankees' loss, Alex Rodriguez spoke about his teammate.
"I saw it all go down," Rodriguez, said. "Obviously it's a huge blow. Mo means so much to this team.
"It's hard to even talk about it tonight. Mo means so much to us on a personal level, and on the field."