By Matthew Geagan, CBS Boston

BOSTON (CBS) — Wednesday night was supposed to be a pretty special night for the Boston Celtics, one that offered our first glimpse of a healthy squad. Brad Stevens had just about every player available to play, and he really took advantage of that fact.

Perhaps a little too much. Stevens emptied his bench in the first half, going with a 13-man rotation that saw just about everyone get some run. On a night when Kemba Walker, Jayson Tatum and Jaylen Brown played together for the first time all season, they shared the floor for just 16 minutes. Walker’s minutes restriction played a part, but Stevens’ college-like approach with his bench did no one any favors.

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A roller coaster game that featured some terrible defense and questionable rotations from Stevens ended with the Spurs walking off the floor with a 110-106 victory. It was a game that the Celtics could have, and really, should have won. Instead, they were left searching for answers.

The Celtics looked fine for the first quarter despite Stevens emptying his bench and even had an eight-point advantage midway through the second. But the team’s rhythm was off and their defense disappeared, leading to one of the worst runs that the Celtics have given up this season.

From that midway point of the second quarter to the final buzzer of the half, the Spurs rattled off a gruesome 29-6 stretch. The Spurs offense was essentially a layup line, and if that wasn’t frustrating enough, the half ended with two Boston turnovers — both by Tatum — that led to four easy points for the Spurs. San Antonio took a 61-47 lead into the locker room.

The ship was righted in the third, with the Celtics outscoring the Spurs by 16 points to give them a two-point lead heading into the final frame. That’s when Stevens really got wild, sending out a lineup of Walker, Marcus Smart, Javonte Green, Grant Williams and Robert Williams. With Walker and Smart the only real scoring options in that defense-heavy lineup, Boston’s two-point lead quickly turned into a three-point deficit.

That five-man lineup shot 1-for-9, and any momentum the Celtics took into the quarter was gone. The Spurs rattled off an 11-2 run, and the Celtics were fighting an uphill battle the rest of the way. When they had a chance to tie or win it in the end, they let that go by the wayside as well, with Dejounte Murray picking Walker’s pocket with 20 seconds left and coasting in for an easy dunk, giving San Antonio a 107-103 lead with 16 seconds left.

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Abysmal defense and odd rotations are no way to win a basketball game. Stevens is much more concerned about the former than the latter, admitting that his lineup tinkering will likely keep going as he tries to determine the pecking order off the bench. But playing 10 guys in the fourth, and 13 overall, kept the team out of sync.

“You can write down all of our bench and I don’t know there has been a ton of separation. I think there will be some,” Stevens said after the loss. “Experimenting and looking for different matches in lineups. We played Grant and Semi in the second when we wanted to be bigger. They bring a physicality to the game and we were playing a lot of zone. They were able to handle the bottom and handle blockouts and also play the perimeter.

“The No. 1 thing in my eyes is we’re not holding down the fort. We’re not guarding the lane, we’re not protecting the rim, we’re not protecting the basket,” he said. “I’m not talking about our bigs; I’m talking about everybody. Everybody needs to protect the rim better. We’re just letting guys lay it in, and these guys are too good if you just let them lay it in.”

The Spurs hit 27 of their 33 shots from the paint. Stevens said he turned to a zone defense out of necessity, “because we couldn’t stop a nosebleed.”

“We want to play multiple defenses and play multiple defenses well, but to do that you have to play one defense well. We have a lot of work to do. That is something that, if we’re going to play small, play big, whatever we play we have to defend at a high level to win,” said Stevens. “There are a lot of things that we have to clean up, but it has to be our DNA, part of our makeup. It is so important to winning, we have to make it even more important on every possession. That stood out, and our lack of ball movement in the first half stood out. Those are the two areas ill take from this game that we really have to improve on.”

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Wednesday night was a frustrating loss that falls on the players, but even more so on Stevens. His experimenting with rotations was to be expected, but he really out-Stevens’d himself in San Antonio, costing his team a very winnable game.