The injury did not occur when the 12-time All-Star was on the mound or in a game. It happened before the Yankees 4-3 loss in Kansas when Rivera was catching fly balls in the outfield during batting practice.
The 42-year-old pitcher apparently caught his cleat where the grass meets the warning track. His right knee buckled just before he hit the wall and fell to the ground, grimacing in pain.
He was circumspect about his future in the game as he faced surgery on the torn ligament and a six to eight months of recovery.
"At this point, I don't know," he said. "Going to have to face this first. It all depends on how the rehab is going to happen, and from there, we'll see."
Yankees fans across the city were were devastated about the freak accident, lamenting the loss of Rivera, considered baseball's greatest closer of all time.
"He was just shagging [fly] balls, which he’s probably done for 20 years," Mayor Michael Bloomberg said during a radio appearance Friday. "Your whole career in one step. But that’s the business.”
He added, "If this ends his career that really is tragic. It’s bad for the Yankees, but it’s terrible for him."
Rivera leads the league with 608 career saves over his 18-year career. He had hinted previously that this year might be his last.
The right-handed reliever is 1-1 and has five saves and a 2.16 ERA in nine games this season.
"He’s got an awful lot to look back on," Bloomberg said. "But I suspect this morning he’s not doing that. He’s looking at his knee and saying, ’Why me?’"