"Mo is a vital part of this team on the field, off the field. He's going to be missed," Yankees captain Derek Jeter said. "There's no other way to put it. You don't replace him. Someone else can do his job, but you can't really replace him."
The Yankees were hitting on the field at around 7 p.m. ET when the injury occurred. Rivera regularly participates in shagging fly balls during batting practice, considering it part of his exercise regimen and one of his favorite times of the day.
Rivera lunged for a ball hit by Jayson Nix near the warning track in left-center field when his knee buckled and the 42-year-old lost his footing, landing on the dirt before crumpling against the outfield fence. Rivera immediately grabbed at his right knee in agony.
"He broke for a ball like he always does, and then it kind of went funny," said Yankees hurler David Robertson, who saw the injury up close. "At first, I thought it was funny -- and then all of a sudden I realized he was injured, he was down. That's when I really got worried. There's nothing I can do except stand there and watch. It's a miserable feeling."
Alex Rodriguez was one of the first Yankees near home plate to notice the closer writhing on the field, saying, "Oh my God," before drawing manager Joe Girardi's attention.
Girardi jogged to the outfield wall with members of the training staff to attend to Rivera, who was gingerly lifted onto a flatbed truck and carted back around the diamond. As the cart rolled away, Rivera waved to acknowledge fans behind the fence wishing him well.
"It's hard to even talk about it tonight," Rodriguez said in a hushed clubhouse. "Mo means so much to all of us on a personal level. Obviously there's a significance on the field, on the mound with his presence. The bottom line is we're the New York Yankees. Nobody is going to feel sorry for us."
Rivera was taken to Kansas University MedWest hospital for an MRI during Thursday's game, which revealed the tear. An initial diagnosis offered the more encouraging term of a "twisted knee," but Royals head team physician Vincent Key confirmed the Yankees' worst fears hours later.
"This is bad, no question about it," Girardi said. "This is not what you come to Kansas City to hear. But good teams find a way to overcome things. If we want to play in October, we're going to have to find a way to overcome it."
The Yankees want to have Rivera travel to New York to be examined by their own physicians, but they are not expecting a miraculous change of fortune. Rivera said he was in no hurry to get back to New York and that he would prefer to stay with the team in Kansas City.
"I just want to be here for the guys, make sure the guys are OK," Rivera said. "It's not an easy situation, but we've been through this before, and we're being tested one more time."
Rivera had strongly hinted this spring that the 2012 season could be his last, saying that he had already reached a decision about his baseball future and that it was "irrevocable." In a twist, he had even spoken about wanting to play center field in a Major League game for one batter before retiring, something Girardi fretted about because of the risk of injury.
But permitting Rivera to be in the outfield with other pitchers was never an issue, Girardi said.
"You have to allow him to be an athlete and be a baseball player and have fun out there," Girardi said. "I've never seen Mo do anything recklessly. I've never seen Mo dive or try to rob a home run, it's one of the ways he exercises. It's really unfortunate."
For most pitchers, the task of "shagging" in the outfield is something of a chore, helping gather up batted balls during batting practice. But Rivera always seemed to enjoy the exercise during his 18-year career, using the opportunity to prove his athletic skills are not limited just to the mound.
"I don't want to have it any other way," said Rivera, who has not been on the disabled list since 2003. "If it's gonna happen like that, at least let it happen doing what I love, you know? And shagging, I love to do. If I had to do it over again, I would do it again. No hesitation. There's reasons why it happens. You have to take it the way it is and fight, fight through it. Now we have to just fight."
Because of the retirement chatter, Rivera's future had been uncertain even before he laid under a Budweiser advertisement on a muggy Missouri afternoon, a billboard reading above him, 'Walk Off A Hero.'
But the Yankees were counting on having his services for at least the rest of this year, a season in which he had already compiled five saves -- the last of which came on Monday against the Orioles in New York -- to push his career total to 608.
Girardi said that he would sleep on how to patch his bullpen together, but it seems likely that Robertson will be pressed into service as the new closer, with Rafael Soriano elevated to eighth-inning duties. The Yankees have to find a way to get by without Rivera, perhaps forever.
"It's crazy. It's tough, but we'll do whatever we can to step up and try to fill the void of the best closer to ever play baseball," CC Sabathia said. "It's going to be real tough, but we have to keep going."