The Celtics announced Rondo's injury during Sunday's 100-98 double-overtime victory against the Miami Heat, stating the four-time All-Star will undergo surgery.
The initial report was that Rondo had a hyperextended knee, but tests showed the ACL tear, an injury from which it usually takes up to a year to recover. A date for the surgery had not been scheduled as of Sunday afternoon, said team spokesman Brian Olive, who confirmed that Rondo's season is finished.
"Obviously the Rondo news is tough," Celtics coach Doc Rivers said after the game. "I knew before the game, but I didn't tell anyone. Obviously it was pretty emotional."
Rondo is expected to seek a second opinion to gauge his best route for recovery. He was chosen as the starting point guard for the Eastern Conference in the All-Star Game in Houston on Feb. 17.
Now the Celtics hope he can be ready for the start of next season.
"How far is that?" Rivers said. "I don't even know how long that is. Eight months? Nine months? So he'll probably be ready, but I don't know."
Celtics forward Kevin Garnett, Rondo's closest friend on the team, took the news hard.
"It was a big blow to everybody in here, me included," Garnett said. "Man, that hurt. It's tough. We had a tough game, came in, and he told the whole team in the locker room. It's tough, tough on everybody.
"(Rondo is) becoming the heart and soul of this team. He's coming into his own. Had some bumps in the road, but we're just trying to be supportive for him."
Rondo, averaging 13.7 points, 11.1 assists and 5.6 rebounds in 38 games this season, was a late scratch Sunday, two days after suffering the injury in a loss to the Atlanta Hawks, a game in which the Celtics squandered a 27-point lead.
Rondo arrived at TD Garden on Sunday expecting to play, but a consultation with the team doctor led him to being scratched and shuttled to New England Baptist Hospital for an MRI.
Courtney Lee started in Rondo's place as the Celtics held on to defeat Miami, snapping their six-game losing streak. Boston (21-23) currently is eighth in the Eastern Conference standings, 2½ games ahead of ninth-place Philadelphia.
Celtics president of basketball operations Danny Ainge said he will "evaluate in the next couple of days" how to fill the void left by Rondo's injury. However, both Ainge and Rivers indicated that Rondo's absence won't prompt Boston to make any major trades.
"We've had success playing without Rajon during short periods of time, but we've never had to play without him during long periods of time," Ainge said. "It should be an interesting test for us -- not a test we wanted. Frankly, I'm worried about that test."
While the Celtics' future is clouded by the loss of their most talented player, Rivers remains confident in his group.
"We'll find someone that's already in our locker room who will step up," Rivers said. "You can write the obituary, but I'm not. We're going nowhere."
As Celtics players walked off the court through a tunnel toward their locker room after Sunday's win, Rondo, dressed in street clothes, greeted them.
"We've just got to rally around each other," Celtics forward Paul Pierce told ESPN immediately following the game. "I feel for him, he was having such a great season. ... Guys just have to step up."
Pierce, who had 17 points, 13 rebounds and 10 assists Sunday, said it was emotionally difficult to learn about Rondo's injury after Boston's thrilling win.
"It's like an upper and a downer," Pierce said. "It's a great win. You get on an emotional high, then you come in the locker room and you see that Rondo's probably going to be out for the season, you've got a lot of mixed emotions in here.
"It's just like you want to celebrate the win, but you feel for your teammate or leader -- a guy who, pretty much, as he goes, we go, and he's going to be out for the year. It's a tough pill to swallow."
Even members of the rival Heat were rattled by the news.
"I think it sucks. It's terrible," LeBron James said. "As much as a competitor and as much as I've been a rival with Boston over the years, I never want to see anyone go down. And knowing the competitor that he is and knowing how talented he is, I think it's terrible, not only for their team but for the league."
Dwyane Wade, who has had on-court flare-ups with Rondo in the past, also was disappointed to hear about the injury.
"We continue to say, one thing about this league is it's a brotherhood," Wade said. "You never want to see anyone go down with any kind of injury, especially a season-ending injury. So it's very unfortunate and it sucks.
"He's a great player and he really is the leader of this team and, obviously, guys are going to have to step up for them. But this year they lost the leader of their team, so it's tough for the game to lose a player like Rondo, but especially for Boston."
ESPNBoston.com's Jackie MacMullan and The Associated Press contributed to this report.