Updated 11:57 PM ET
(CBS/AP) KANSAS CITY, Mo. - New York Yankees manager Joe Girardi says closer Mariano Rivera has a torn right knee ligament, an injury that could potentially end his career.
Earlier, Rivera was carted off the field and taken for tests after twisting his right knee shagging flyballs during batting practice Thursday night.
Baseball's career saves leader was hurt about an hour before the Yankees' game at Kansas City. Rivera was examined by Royals associate physician Dr. Joe Noland and Yankees trainers, and was sent immediately for an MRI exam at KU MedWest.
Rivera was tracking down a ball hit by Jayson Nix in deep center field when his right knee appeared to buckle a step before the wall. He fell to the warning track and immediately grabbed at his right knee, briefly covering his face with his glove and grimacing in pain.
Bullpen coach Mike Harkey was near Rivera when he went down, and he was the first to whistle for help. Manager Joe Girardi was watching batting practice from behind home plate and started running down, cutting across the outfield to get to his closer.
Near home plate, teammate Alex Rodriguez could be seen saying, "Oh, my God. Oh, my God."
Harkey and Girardi helped to carry Rivera to the cart, gently setting him into the back with his knee propped up. The cart rounded the warning track before disappearing up a tunnel.
The 42-year-old Rivera regularly catches balls during batting practice as a way to keep in shape, and has jokingly said he'd like to play in the outfield before he retires.
He's dealt with inflammation in his right knee in the past.
Rivera (1-1) blew the save on opening day, but he's only allowed two hits in eight scoreless innings since then, picking up five of his 608 career saves.
The remarkably durable reliever hinted during spring training that this could be his final year in the major leagues, but also said that he won't announce his long-term plans until after the season. His last trip to the disabled list came in 2003, when he had a groin strain.
Rivera hasn't shown any effects of age, either. He's made at least 60 appearances each of the last nine seasons, setting a standard for bullpen excellence that may never be touched.
He appeared in 64 games last season, and his 1.91 ERA was among the best of his 18-year major league career. Rivera wound up saving 44 games, tied for his most since the 2004 season.