Things unraveled for the Celtics both on the court and off it afterward.
The Celtics were in control of the game when Irving hurt his knee with about five minutes left in the second quarter while attempting to defend Garrett Temple at the top of the key.
The Clippers outscored the Celtics 75-44 after Irving left, coming back from 28 points down to win 123-112.
It was the largest comeback in the NBA in the past two seasons and the largest in Clippers history. It was the biggest blown lead by the Celtics since Dec. 5, 2003. And it came two nights after Boston gave up an 18-point lead to the Los Angeles Lakers at TD Garden.
Celtics coach Brad Stevens said Irving's knee appeared to "buckle" in the second quarter and that the 26-year-old will undergo more tests on Sunday.
But Stevens said the injury wasn't to blame for his team's collapse.
"There's no excuses," Stevens told NBC Sports Boston. "Those things happen and we all have to pick up for that. So, I'll go back and figure out what I can do better, what we can do better, and we'll go from there."
Boston's Marcus Morris wasn't as measured in his comments afterward.
The veteran forward blamed the loss on the team's lack of cohesion, calling the Celtics' effort "unacceptable" and saying Boston hasn't had fun while playing in a long time.
"I mean, for me, it's not really about the loss," Morris said. "It's about the attitude that we're playing with. Guys are hanging their head. It's just not fun, it's not fun. We're not competing at a high level. Even when we're winning it's still not fun. I just don't see the joy in the game.
"I watch all these other teams in the league, guys up on the bench, up on the court; they're doing stuff that looks like they're enjoying their teammate's success, they're enjoying everything and they're playing together. And when I look at us, I just see a bunch of individuals."
Morris said there are no more excuses with the trade deadline behind them.
"For us to be a championship team, man, this s--- has to change," he said. "We have to genuinely want to win. That has to be the first goal. Whatever it takes, I'm with it. If you want to take me out the lineup and get some juice in it, I'm with it. But I'm trying to win. That's the goal. Trade deadline's over. This is the team we're rocking with. This is what we're going with. So, at the end of the day, man -- it's unacceptable what we did tonight. It's unacceptable for what we did with a team of our caliber."