New York Yankees closer Mariano Rivera, Major League Baseball's career saves leader, may be lost for the season after tests showed he had torn a knee ligament, manager Joe Girardi said on Thursday after their game in Kansas City.

Rivera, 42, hurt his knee while fielding fly balls during batting practice before the game against the Royals.

"You lose a Hall of Famer ... if that's what it is, that's as bad as it gets," a sombre Girardi told reporters after New York's 4-3 loss.

The Panamanian right-hander, who has registered a record 608 career saves, suffered the injury while running back in deep center field to chase down a ball hit by Jayson Nix.

It could also mean the end of Rivera's illustrious career. The closer said before the season he had made up his mind whether this campaign would be his last but was not going to reveal his decision until after the season.

Rivera's right knee buckled as he approached the warning track going after the fly ball and collapsed to the ground near the fence, clutching his knee in pain.

The reliever was at first diagnosed with a strained knee after being examined on site at Kauffman Stadium by Royals associate physician Joseph Noland and several members of the Yankees training staff.

An MRI scan performed at a nearby hospital revealed the torn ligament, Girardi said.

Rivera, who set the league's saves record with 602 against the Minnesota Twins in September, regularly chases fly balls as do many pitchers during batting practice as part of their conditioning.

Rivera, who has also posted a record 42 postseason saves, had shown no signs of slowing down, registering a 1-1 record with five saves and a 2.16 earned run average this season.

"There's reasons why it happens," an emotional Rivera told reporters. "You have to take it the way it is and fight through it. Now we just have to fight."

(Reporting by Mark Lamport-Stokes in Los Angeles and Larry Fine in New York; Editing by Ian Ransom)