The Celtics (28-5) are still well out in front in the Atlantic Division and lead Cleveland (26-5) by one game for the best record in the Eastern Conference.
Paul Pierce scored a game-high 28 points while Kevin Garnett had 17 points and 8 rebounds. But the Celtics shot only 40.3 percent from the field. Ray Allen had 12 points on 2-of-11 shooting, and the Boston bench had 9 points and 2 rebounds.
"There is nothing to hang our head about," said Kendrick Perkins. "We just need to keep working."
Said Pierce, "It's a tough one to swallow, but it's a long season."
The Blazers (20-12) were without guard Brandon Roy, who averages a team-best 23 points per game, because of a right hamstring injury.
Rookie guard Rudy Fernandez made his first NBA start in Roy's place.
Despite a left ankle injury, Portland center Greg Oden had a solid game with 13 points and 11 rebounds. Blazers forward LaMarcus Aldridge had 20 points and 7 rebounds, while guard Steve Blake added 21.
"Roy was holding the Trail Blazers back," joked Rivers. "Sorry, I had to laugh at something."
Said Portland coach Nate McMillan, "This was a great win for us, and now we have to build on that. Before the game it was no secret what we had to do. They don't beat themselves. You have to beat them."
The Celtics were upset about a play that cost them 2 points in the first half after an odd call.
Portland's last bucket of the half came on a true power-play dunk by Travis Outlaw - the Blazers had six players on the floor. A technical was called, with Allen making the free throw, but Outlaw's basket was allowed to count.
"That was awful," Rivers said. "It absolutely had an affect on our guys. They complained the rest of the game about it. I told them, 'You got to get over that.' That was a doozy. There is no excuse for that to happen.
"They said you couldn't correct the play, which I still disagree with. But the problem was that we called around, we called the league, and they didn't have the answer, either. It is what it is. There is nothing we can do about."
Said referee Mike Callahan, "If we would have caught the six men on the court before the made field goal, then there would have been no score. We would have called a technical foul on Portland and stopped play.