New York (CNN) -- Flanked by reporters inside a clubhouse in Kansas City, the New York Yankees' Mariano Rivera sought to quiet speculation over whether he would ever take the mound again.
"Definitely I plan on coming back," said the 12-time All-Star reliever, who tore his anterior cruciate ligament, or ACL, on Thursday while shagging fly balls.
An emotional Rivera responded to a reporter's question the day of his injury about whether New York fans would every see another late-inning performance from the aging right-hander. "At this point, I don't know," he said.
But a day later, Rivera, baseball's all-time leading career saves leader, seemed a little more certain.
"I'm not going down like this," he said Friday at Kauffman Stadium, where his club is in a four-game series against the Royals.
Rivera's injury is seen as a potentially career-ending setback for the 42-year-old reliever, who is widely regarded by analysts as among the most dominant closers in baseball history.
Recognized for his cut fastball, or "cutter," a slider-like pitch that breaks late as it approaches the plate, Rivera's 2.21 career earned run average (.70 in the postseason) is the lowest since 1920 among pitchers with at least 1,000 innings under their belt.
Moments after Rivera collapsed on Thursday, Yankees third-base man Alex Rodriguez could be seen mouthing the words "Oh my God," before bullpen coach Mike Harkey and manager Joe Girardi ran over and carried Rivera off the field.
Nicknamed "Mo," the Panama native drew support from teammates during a press conference on Friday.
"Mo's a vital part of this team," said Yankee captain Derek Jeter.
"He's going to be missed. There's no other way to put it," Jeter told reporters. "Someone else can do his job, but you can't really replace him."
Yankee reliever David Robertson, Rivera's likely replacement, described the 18-year veteran as "the anchor of our bullpen."
"I'm not Mariano Rivera, I'm not going to be able to go out there and do exactly what he does, get outs real quickly, I'm a different style pitcher," Robertson told reporters. "I've never done it, so all I can do is say I'm going to try real hard."