Garnett was suspended for one game for hitting Milwaukee's Andrew Bogut in the face near the end of Saturday's game.
"I was surprised. I thought it was a ridiculous suspension," Rivers said before Tuesday night's game against the Knicks. "I just want more consistency from our league."
Rivers said Garnett was hit in the face twice from the side and he flailed out an arm as a reflex.
"I guess that constitutes a punch," the coach said.
The suspension didn't cost the Celtics on Tuesday, however, as Boston escaped the Knicks 110-101.
Information from The Associated Press was used in this report.