It is the first time since the 2007-08 season that the Celtics are in sole possession of first place in the East this late into the season. It's also the first time since Nov. 17 that the reigning champions haven't sat atop the conference.
After the game, the Cavaliers mostly brushed off questions about the importance of securing the top seed in the East, saying it's far more important to play better than they have, as they've gone just 7-10 since Feb. 25.
"That's fine," Cavaliers star LeBron James said when informed Boston had passed Cleveland in the standings with the 103-74 loss. "It matters more that we play better basketball than where we're at. If that results in the 1, 2 or 3 seed, we need to play better basketball. That's all it comes down to. I'm not worried about anything."
The Cavaliers were downright awful Monday, scoring a season-low 74 points on just 15.4 percent shooting from 3-point range.
Cavaliers coach Tyronn Lue said he thought his team looked flat and tired against San Antonio, which improved to 4-0 versus the Golden State Warriors and Cavs this season. The four wins have come by an average of 20.8 points.
"The crazy part about it -- we start off the right way, but teams are so much faster than us," Lue said. "They just look so much faster than we are. I mean, damn. It just looks like -- just beat us on the dribble, transition, just looking faster."
The Cavaliers had a 5½-game lead on the Celtics on Jan. 10. Since then, Cleveland has gone 19-17 while Boston has gone 25-11.
"We're doing the right things, trying to do the right things, but it's been tough on us," Lue added. "We score 74 points and shoot 4-of-26 [on 3-pointers] -- we don't even really give ourselves a chance. I got to watch the film. I can't fault the guys for not trying. I just thought we couldn't do it."
Lue acknowledged he had considered resting key players for a block of time at the end of the season, in much the same way his mentor in Boston, Doc Rivers, did with Kevin Garnett, Ray Allen and Paul Pierce heading into the 2010 playoffs.
"It's crossed my mind," Lue said. When asked to elaborate on which players might sit and when, he said, "I don't know, I might get in trouble. ... I know a lot of guys are tired. With LeBron and Kyrie [Irving] having to carry the load, Kevin [Love] and JR [Smith] being out so long, they're pretty worn down.
"But that's no excuse. A lot of other players are tired, too. When you've got one of the top-three point guards in the league and the best player in the world, that's your job. You're supposed to carry us and do what you've been doing."
For his part, James said he felt "great" physically. But he agreed with Lue's assessment that the Cavs seem slow right now, and might have heavy legs due to fatigue.
"That's an individual question. But at the same time, yeah, we look a little slow versus a lot of these teams," James said. "It's up to the individual where if you're feeling a little slow, OK, you gotta get in better shape.
"It sucks to say, 'It's March, you're not in good shape. But you gotta get in even better shape against some of these teams that's been plugging away all season long.' I know from a personal standpoint, I feel great. I feel really good, actually."
This isn't the first time Lue has said other teams seem to be playing faster than the Cavs. Asked whether that's a function of tired legs or a lack of athleticism, Lue said it was probably both.
"We do have slow guys," Lue said. "And guys are tired. Games that we would normally try to get those guys some rest, it just hasn't been happening.
"But I promise, we're going to be fine. I'm not pressing. They know that, we know it. We just have to play better."
According to data from SportVU, the Spurs may be the second-oldest team in the league, but on offense they rank as the fifth-fastest team in the league, while the Cavs rank last in the league. On defense, San Antonio is the third-fastest team in the league, Cleveland is 26th.
Said James, " When you've been struggling, this is the last team you want to play -- a well-oiled machine like this. They exploit everything that you might be doing wrong at this point in the season, and right now, we're just not playing good basketball."
James said he didn't think the Cavs' schedule or rash of injuries -- Love is still coming back from knee surgery, Iman Shumpert (knee) and Kyle Korver (foot) sat out Monday -- should be used as an excuse.
"I hate excuses. It's not what I'm about. It's never been what I've been about. I don't like talk about injuries, I don't like talking about the scheduling or things of that nature," he said. "Listen, the schedule is the schedule. We got 17 games this month. We've been in six different time zones in the last eight days. We have to try to fight through it. If you're going to have heavy legs, you're going to have heavy legs. That's why we have a group of 15."
However, if a teammate did need time to recuperate, James said they should talk to Lue about it.
"Coach is going to have his logic of things, but at the end of the day we need to play," James said. "If guys are having heavy legs, then they should go to Coach and say they need one."
Cavaliers guard Irving ran back onto the court to work on his shot postgame after struggling from the field against the Spurs; he scored just eight points on 4-of-13 shooting.
Irving's minus-29 plus-minus rating is tied for the lowest of his career and his lowest since 2014.
Irving sat with James for an extended emotional conversation at his locker after the game but did not speak with reporters before boarding the team charter to Chicago. The Cavs will have two days to rest before playing the Bulls on Thursday.