Mariano Rivera's career may have ended Thursday night, not while basking in the glow of adoring fans at Yankee Stadium, but in agonizing pain on the outfield grass before a few thousand fans in Kansas City.

Baseball's greatest closer tore a ligament in his right knee while shagging balls during batting practice before a game against the Royals, a devastating injury that will likely sideline the Yankees' 12-time All-Star for the remainder of the season.

"This is bad," Yankees manager Joe Girardi said. "There's no question about it."

The 42-year-old right-hander's leg caught on the field where the grass meets dirt, causing his knee to buckle. He fell into the outfield wall and down to the ground, where Rivera grimaced in pain as teammates and training staff ran out to see him.

Rivera was carted from the field and taken for an MRI exam. Royals physician Dr. Vincent Key diagnosed a torn ACL after examining the scans of the knee.

"I thought it wasn't that bad, but it's torn," Rivera said after the Yankees lost 4-3, pausing several times in the Yankees clubhouse to compose himself. "Have to fix it."

Rivera has said that he will decide after the season whether to retire, and while Girardi said he hoped that baseball's career saves leader with 608 would make a comeback, Rivera was noncommittal.

"At this point, I don't know," he said. "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."

The injury cast a pall over the Yankees, who put the tying run on third base in the ninth before Mike Moustakas made a stellar play on a chopper by Alex Rodriguez, throwing him out at first to preserve the Royals' victory.

Afterward, the only thing on A-Rod's mind was Rivera.

"I saw it all go down," said Rodriguez, who uttered "Oh, my God," from behind the batting cage when Rivera went down. "Obviously it's a huge blow. Mo means so much to this team.

"It's hard to even talk about it tonight," Rodriguez said. "Mo means so much to us on a personal level, and on the field."

Bullpen coach Mike Harkey was near Rivera when he went down, and was the first to whistle for help. Girardi was watching batting practice near Rodriguez behind home plate and ran down the third base line before cutting across the outfield to get to his closer.

Harkey and Girardi helped 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 initial diagnosis was a twisted knee, but Girardi had a feeling the injury was much worse when he didn't receive word as the game pressed on.

"My thought was he must have torn the ligament, the way he went down," Girardi said.

Girardi was quick to defend Rivera's decision to shag balls in the outfield, pointing out that he may never have become a five-time World Series champion without putting in such work. He called it a fluke injury, not unlike somebody falling off the curb or down the stairs.

"You've all seen Mo run around here for what, 40 years?" Girardi said.

Derek Jeter said that Rivera has shagged balls for the "20-something" years that he's known him, and never once did the notion that he could be hurt cross his mind.

"It's bad. There's no other way to put it," Jeter said. "It's just a freak thing."

Rookie starter David Phelps (0-1) said he thought for a moment that Rivera was just joking, but once training staff gathered around the closer, he knew something bad had happened.

"There's nothing I can do but stand there and watch. It's a miserable feeling to see it," Phelps said. "I didn't think it was that bad. I was just hoping he caught it funny or sprained it or something, and then we came in here after the game and found out the news."

The outcome of the game was almost secondary for the Yankees, though that was hardly the case for Kansas City, which snapped a 10-game home losing streak.

Danny Duffy (2-2) went six strong innings before turning it over to the bullpen, and three relievers maintained a one-run lead until Jonathan Broxton came on to close the ninth.

He allowed a leadoff single to Jeter, his fourth hit of the night, and then walked Curtis Granderson. Mark Teixeira followed with a liner to second base, but Chris Getz snagged it in the dirt and fired to Alcides Escobar covering second to start a double play.

Jeter advanced to third for Rodriguez, who hit a chopper to Moustakas at third base. He made a barehanded grab and threw out A-Rod by a step to preserve the victory.

"It's a load off," Royals manager Ned Yost said. "We don't have to talk about not winning games at home anymore. That's a good thing. Hearing the fans in the ninth inning was like music to my ears because, I haven't heard that all year long and this was the 11th game."

Moustakas homered in the second and added a two-run single in the fifth.

"Moose is just on fire with the bat right now, three RBIs, the home run to give us the first lead and the big two-run single there was a big boost for us," Yost said.

The Royals were the third team in major league history to lose their first 10 home games when they dropped every one during their first homestand. They started to turn things around on a rain-shortened 4-3 road trip, and kept the momentum going against the struggling Yankees.

Even in the Royals' clubhouse, though, Rivera was on everyone's mind.

"That's horrible news," Broxton said. "As many saves as he's been out there and as good an athlete as he is, I just hate for bad news. All I can do is wish him the best."

Notes: Jeter is batting .404 this season. ... The Royals placed Yuniesky Betancourt (right ankle sprain) on the DL purchased the contract of INF Irving Falu from Triple-A Omaha. ... Yankees 3B Eric Chavez was released from the hospital after having concussion-like symptoms Wednesday night. He was put on the seven-day concussion DL and Jayson Nix signed to a major league contract. ... LHP CC Sabathia will pitch Friday night for New York. LHP Bruce Chen goes for Kansas City.