Rondo was banned one game without pay Monday for making contact with a referee and failure to cooperate with an NBA investigation.
With 3:19 remaining in the third quarter of the Celtics' 89-81 victory over the Atlanta Hawks on Sunday, Rondo was called for an offensive foul on a drive to the basket. He went up to referee Rodney Mott and bumped into him as the two walked back up the court.
Rondo missed the Celtics' game against the New York Knicks.
"I saw it when it happened. I didn't think anything of it honestly, and obviously the officials didn't either, they didn't call a tech," Rivers said before the game. "But you know the old saying is you're not given a reputation, you earn one, and I'm sure that had a lot to do with it as well."
Rondo was suspended in November for two games for shoving Brooklyn Nets forward Kris Humphries, and drew a suspension during last year's playoffs for bumping a referee. Rivers said Rondo has to improve his reputation within the league.
"We just work on it, every day," he said. "He's going to play a long time and he has a lot of time to change it."
Rivers said Rondo had a brief conversation with a league official, and wasn't sure how much that contributed to the decision to suspend.
"Usually when they call, it usually means the decision's made pretty much for the most part," Rivers said. "But listen, all of it is disappointing."
Jason Terry started with Avery Bradley in the Celtics' backcourt.