KANSAS CITY, Mo. -- The incomparable career of New York Yankees closer Mariano Rivera may have ended Thursday night, not on a pitcher's mound but on the warning track at Kauffman Stadium before the game had even begun.

Rivera, 42, the most successful closer of all time, suffered a torn ACL and a torn meniscus in his right knee after he fell awkwardly to the ground while shagging a fly ball during batting practice before the Yankees' game with the Kansas City Royals.

The 12-time All-Star was lifted by manager Joe Girardi and a coach and placed on a cart, driven off the field, and needed assistance to walk into the clubhouse. Then he was whisked to a nearby hospital for an MRI, which revealed the news.

And when he was asked some four hours after the injury and minutes after the Yankees had lost 4-3 to the Royals if he thought he could come back from the injury, which is likely to require season-ending surgery, Rivera said, "At this point, I don't know. At this point, I don't know.

"Going to have to face this first. It all depends on how the rehab is going to happen, and from there, we'll see."

He then fell silent, overcome with emotion and unable to speak.

Asked to describe his pain, Rivera said, "It's more mental than physical right now. I let the team down."

And he fell silent again, his eyes brimming with tears.

Rivera was injured while leaping for a ball hit by Jayson Nix, the newest Yankee who joined the team at about 3 p.m. Thursday.

Rivera, who regularly catches balls during batting practice as a way to stay in shape, fell to the warning track. His face was contorted in obvious pain, and he was rubbing his right knee with his right hand.

"I got myself between the grass and the dirt, and I couldn't pull my leg up and twisted my knee. ACL. Torn. Broken. Meniscus, also," said Rivera, who has won World Series and LCS MVP awards. "I thought it wasn't that bad. I mean, I was walking a little bit. But, uh, it's torn. I have to fix it."

Yankees shortstop Derek Jeter, Rivera's teammate for 18 seasons with the Yankees,

was optimistic that Rivera could return.