From rushed shots to unnecessary fouls to missed free throws to losing the battle on loose balls, Rivers was baffled by his team's inability to make quality decisions and execute the most routine fundamentals Friday night.
The result? A head-shaking 119-114 overtime loss to an injury-ravaged Houston Rockets squad that really had no business taking the game to overtime, let alone emerging with a victory at TD Garden.
"The other stuff, whatever, but we've been a pretty smart team, and tonight, I thought it was April 1," Rivers said, referencing April Fool's Day. "It's amazing how many things we did that were out of character, and that bothers me. I can live with the loss, but when you don't play right, that bothers me."
The Celtics almost could write off their last two losses. The San Antonio Spurs ran Boston out of its own gym on one of those shrug-your-shoulders nights that every team endures over the course over an 82-game season. The Oklahoma City Thunder benefited from a hefty free-throw disparity and Boston at least could lean on that excuse.
But Friday didn't boast an obvious silver lining for a Celtics team that has lost three straight heading into an Easter showdown against the Eastern Conference-leading Cleveland Cavaliers on Sunday.
The Celtics have been playing the "we still have X games to get this right" card since before the All-Star break. But now they're down to seven games, and there's seemingly a new hole to plug after each game.
There certainly were Friday.
"So many [mistakes] that it will be an interesting film session [Saturday]," Rivers said with a sigh.
The Celtics had numerous opportunities to put this game away. Boston went on an 11-0 run to open a seven-point cushion with less than six minutes to play and gave it away.
The Celtics missed three free throws over the final 46 seconds of regulation with a chance to salt away, and Aaron Brooks drilled a 3-pointer to force overtime (once Paul Pierce missed with a chance to win in regulation).
Boston opened a five-point lead in overtime, then let the Rockets score the final 10 points of the game.
The Celtics played, in the words of Rivers, "like a high school team," not a veteran group that oozes championship experience.
"It's very frustrating," said resident straight-shooter Kendrick Perkins, who scored 15 points on 7-of-10 shooting but watched Houston control the glass late in the game. "I feel like we've got to execute better down the stretch. Be smart. We're a veteran team. We can't just do dumb stuff down the stretch. We have to get quality shots and don't give up the ball."
Perkins wasn't exempt from the dunce cap. Boston had a chance to tie the game in overtime after forcing a turnover with 14 seconds to play. But Perkins got whistled for an illegal screen trying to free Michael Finley for a potential tying 3-pointer. Forced to foul, the Celtics watched Brooks put the game away from the line.
"It's very uncharacteristic; it's not like us," Perkins admitted. "We've got seven games to get it right. As far as team defense goes, it starts individually."
Defense is the biggest concern, considering it had been Boston's hallmark over the last two seasons. Offensively, the Celtics dominate points in the paint, but Friday, as they were against Oklahoma City, were woeful defending the perimeter.
Captain Paul Pierce went out of his way to stress that dribble-drive penetration killed the Celtics, as driving guards were kicking to the corners for open 3-pointers.
Houston finished 12-of-18 from beyond the arc, a 66.7 percent clip that was better than what Boston shot at the free-throw line (24-of-37, 64.9 percent).
But Rivers said it wasn't as simple as poor 3-point defense.
"You can also say the difference was we missed 13 free throws," Rivers said. "But I didn't think we played very smart tonight. There are so many little things that I could point out -- that I won't -- but throughout the game, taking shots when you should take the final shots at the end of quarters, and then it leads to them getting a 3-point play, fouling in penalty when you're up late in the game and there's no need to foul. We played like a high school team at times, as far as the way our thought process was.
"I thought, offensively, obviously you can't argue with 52 percent [shooting and] 114 points. And 37 free throws. You're happy with all that. But I just thought there were so many little things.
"We should have put it away, we had chances, we had great shots. We missed a ton of layups down the stretch, close shots. But there were so many little plays to me. No matter if you're playing the Rockets or Cleveland -- it doesn't matter who you are playing -- you have to make [those plays] that a veteran team should make. And we didn't make them. So that was a disappointment."
Chris Forsberg is the Celtics reporter for ESPNBoston.com. Follow him on Twitter.