"I'm coming back," Rivera said. "Write it down, in big letters. I'm not going out like this."
With that statement from No. 42, the question now turns from one of if to a matter of when.
Typically, there's some sort of comparison that can be made to another player's return from a similar injury, but that's where it gets complicated with Rivera. Knee injuries, in general, aren't all that common in pitchers, let alone ACL tears.
One case that can be used as some sort of barometer is that of Brewers starter Yovani Gallardo. The right-hander tore the ACL in his right knee in a collision at first base on May 1, 2008, and -- after having surgery to repair the torn ligament -- returned to make one regular-season start in late September and two postseason appearances.
Gallardo, however, was 22 at the time, making him almost half the age of the 42-year-old Rivera.
"It's tough. It's a lot of hard work," Gallardo said. "When I had to go through it, it was nonstop, every day, rehabbing every day for 2 to 3 hours or whatever it might be. For myself, I had the opportunity to come back and pitch that same year, which was unbelievable. I never thought that [would happen]. Honestly, I didn't. I thought I was going to be done for that whole year.
"To have the opportunity to come back and have a start before the year was over, and then start in the postseason, that was pretty good."
One thing Gallardo and Rivera share in common is that both players sustained the injury to their right knee -- a silver lining of sorts, according to Dr. Neal ElAttrache of the Kerlan-Jobe Orthopaedic Clinic.
"No, it's not guaranteed," said Dr. ElAttrache when asked if a pitcher in Rivera's situation would miss the entire season. "The thing he's got going for him is that it's his drive leg, his push-off leg -- not his plant leg. So if he got it done [surgery] and was able to rehab the power drivers -- his glutes and his quads -- quickly and didn't have much pain, then it's possible that he could be playing, potentially, at the end of September, if needed.
"I think that's a possibility, but there would be as many, or more, guys that wouldn't be ready to play by the end of the season than would. But because it's the right leg, if he's able to get his power back, it's not out of the question."
There are three other players currently on the disabled list as a result of ACL tears -- Tigers catcher Victor Martinez, Brewers first baseman Mat Gamel and Athletics third baseman Scott Sizemore. While Sizemore and Gamel are expected to miss the remainder of the season, Martinez -- who sustained his injury in mid-January -- holds out hope that he will be able to return in the later stages of the season.
As for the age factor, Rivera could look to Braves third baseman Chipper Jones as a model for a veteran player who returned from an ACL tear. Jones underwent surgery on Aug. 14, 2010, at the age of 38. On Opening Day the following year, he went 2-for-4 with a double and scored the eventual winning run from second base on a single.
Jones said on Friday that he'd embrace the opportunity to help Rivera through the recovery process if he has any questions.
"I feel for anybody who blows an ACL because your year is over," Jones said. "You're looking best-case scenario at five or six months. It's a shame because Mo and I are coming down to the end. I doubt very seriously that he's thrown his last pitch. But you hate to cheat the fans from getting to see a guy like that pitch. I wish him the best. If he needs any advice, I've been through it."
As vital as the physical rehab process will be for Rivera, the Majors all-time saves leader, though, fellow closer Joe Nathan insists the mental aspect will be just as significant in determining when the next "Enter Sandman" track will blare over the Yankee Stadium speakers.
While Nathan can't speak to Rivera's specific injury, the Rangers closer is no stranger to going under the knife.
"This is going to be more about desire than physical," said Nathan, who missed the entire 2010 campaign after undergoing Tommy John surgery. "The physical thing is nothing if you've done your work and all the things that you're supposed to do over the course of your career. If you do that, you can get through the physical thing.
"He's in great shape because he has taken care of himself. It's a matter of where he is mentally and his desire ... does he have the desire to go through the rehab part and play again. I'm not speaking for him, but it's on him and what he wants to do. Physically it's not going to stop him."
Rivera answered to the desire aspect on Friday, emphatically stating that, whether in the midst of a potential playoff run later this summer or Opening Day next spring, he will again toe the rubber for the Yankees. For now, he plans to see a doctor on Monday in New York and expects to undergo surgery in the near future.
"I love to play the game, and I don't think, to me, going out like this is the right way," Rivera said. "Everything is going to be fixed."