BOSTON -- Ray Allen’s return to TD Garden became a secondary story Sunday when news broke that Celtics guard Rajon Rondo would miss the rest of the season with a torn ligament in his right knee.

Rondo apparently tore his anterior cruciate ligament during the Celtics’ loss to the Hawks on Friday night. After experiencing persistent discomfort in his knee since Friday, Rondo was evaluated by trainers before Sunday’s game and then taken to a hospital for an MRI to determine the injury.

“Obviously, the Rondo news is pretty tough,” Celtics coach Doc Rivers said. “I knew it before the game; no one else knew it. I just didn’t think it was any time to tell any of our guys that. I told them after the game.”

The Celtics’ postgame locker room was understandably subdued despite their first victory in seven games.

“It was a great win,” Paul Pierce said. “You’re on an emotional high in the locker room and then you see that Rondo is probably going to be out for the season. You have a lot of mixed emotions. … You want to celebrate the win, but you know you feel for your teammate, a leader.”

Boston is currently in eighth place in the Eastern Conference standings, and losing the East’s best point guard could scuttle the season for the Celtics. On Sunday, reserve guard Leandro Barbosa helped pick up the slack, scoring nine points to go along with four assists in 30 minutes.

“I still like this team,” Rivers said. “Obviously, that’s a blow. It’s a huge blow for us. Listen, when something like this happens, someone will find someone that’s already in our locker room that’s going to play terrific. And I have no idea who it is. Tonight it was [Barbosa] and the team. And that’s what we’re going to have to do.”

Heat coach Erik Spoelstra called Rondo’s season-ending injury “awful,” and LeBron James described the news as “terrible.”

“It sucks,” James said. “As much as a competitor and as much as a rival I’ve been with Boston and Rondo over the years, I never want to see anybody go down. And knowing the competitor that he is, how talented he is, I think it’s terrible not only for their team but for the league.”

Allen, Rondo’s former teammate, said the injury was “unfortunate.”

Jeers and cheers

It started with booing and ended with a standing ovation.

TD Garden played a video montage of Allen’s best moments in Boston during the first quarter. The reaction of the Celtics fans to the tribute encapsulated their feelings toward the NBA’s three-point king: frustration and appreciation.

At the beginning of the video, boos loudly rained down on the court as Allen’s past three-pointers and smiles flashed on the videoboard. Then something special happened. Cheers and claps gradually began drowning out the heckling.

“When I saw it, all those moments came streaming back from all the great things that we did here,” Allen said. “I will always remember the big games we played in and the big games we won. I always know that I will always be a Celtic in my mind, regardless of what anybody else says.”

By the end of the video, the entire arena was on its feet and cheering for Allen.

There are still plenty of hard feelings in Boston over Allen leaving for the rival Heat, but not so much that fans no longer respect what Allen did for the franchise.

The video panned to a shot of the championship banner Allen helped win, and the crowd began its standing ovation. Despite the cheers during the video, Boston fans still booed Allen every time he touched the ball during the game.

“I didn’t know what to expect,” Allen said. “An early game is always tough regardless of the circumstances. I didn’t expect to get booed throughout the whole game every time that I touched the ball. That was interesting.”