Then he came to the defense of the Bucks and coach Terry Stotts, who became the object of fans' ire after yet another blown second-half lead in a 111-98 Boston victory on Saturday night.
Paul Pierce scored 34 points, including 16 in the third quarter, as the Celtics overcame a 14-point halftime deficit to take the lead at the end of the third quarter and cruise to victory.
In the past week, Milwaukee blew second-half leads in two separate games against Indiana.
"You guys have got to give them a break," Rivers said. "Honestly. They have key injuries."
Milwaukee came into Saturday's game missing two starting forwards: Bobby Simmons, who has a heel injury, and major offseason acquisition Charlie Villanueva, who remains sidelined with a torn left elbow ligament.
"Take two stars off of any team, and I'll show you a bad basketball team," Rivers said.
It'd be hard to argue with that on Saturday, but Stotts said injuries aren't an excuse.
"At some point, we've got to learn how to play consistently, rather than playing hard when we're down 20 or playing hard to (catch) up," Stotts said. "It's a matter of playing with the same energy, the same emotion, the same execution."
With the Celtics leading 97-87 late in the fourth quarter, someone in an otherwise quiet Bradley Center crowd yelled "Fire Stotts!" A handful of fans picked up on the sentiment, but most stayed quiet -- or left early.
It wasn't the ideal way for Stotts to celebrate his 49th birthday.
Bucks guard Michael Redd -- one of the only players present in an otherwise desolate Milwaukee locker room after the game -- said he understood the fans' frustration but that Stotts still had the team's full attention.
"I believe so," Redd said. "I won't allow it to go any other way."
Things don't get any easier for the Bucks, who must face the Los Angeles Lakers, Phoenix, Golden State and Utah on a four-game western road trip.
"We've got more to worry about than a four-game road trip," Stotts said. "We've got to get back on track."
Rivers praised Pierce for remaining a scoring threat despite drawing a tough defensive assignment in Redd, who led Milwaukee with 24 points.
"You're never going to stop him, you just want to limit his touches," Pierce said.
Pierce said the victory was an important step toward regaining momentum. Boston was coming off two straight losses, including a blowout at home by the New York Knicks on Friday night.
"When you win, you gain confidence," Pierce said. "You try not to get overconfident. I keep telling those guys it's a long season."
The Bucks held a 60-46 halftime lead Saturday, but the Celtics rallied in the third quarter to tie the game at 79 on a layup by forward Ryan Gomes.
Bucks guard Mo Williams then was sent tumbling to the ground after an apparent hard foul by Sebastien Telfair, but Williams was called for traveling. Pierce scored on a drive on the Celtics' next possession, putting the Celtics ahead 81-79 with 55 seconds left in the third quarter.
After Redd hit one of two free throws, Delonte West hit a long jump shot at the buzzer to take an 83-80 lead at the end of the quarter.
"I don't know how to describe it," Stotts said.
Milwaukee's defense continued to break down in the fourth quarter as the Celtics pulled away.
Wally Szczerbiak added 21 for Boston, and Gomes scored 15.
"We took this as a must-win game," Telfair said. "We looked at our division and where we're placed at right now. Coming into Milwaukee against a team that's playing well with great players, we went out there with a sense of urgency to win the game. We had fun in the fourth quarter, but still won the game."
The Bucks had snapped a three-game losing streak with a victory over Philadelphia on Wednesday, but lost out on their chance at their first two-game winning streak of the season and have now lost four of their last five.
Andrew Bogut added 18 points.
Milwaukee guard Mo Williams, who sat out the Bucks' victory over Philadelphia on Wednesday with an elbow contusion, started on Saturday. ... Celtics coach Doc Rivers, a former star at Marquette, called Golden Eagles coach Tom Crean earlier in the day on Saturday.