Garnett will be shelved for at least awhile after aggravating the injury during a Friday game against Dallas, the coach told the Celtics' official Twitter feed.
When asked if he thought Garnett's prognosis from a magnetic resonance imaging exam was "serious," Rivers replied, "If you mean that he's not going to play this year, no, I don't worry about that. Can he miss 4, 5 games? Yeah."
The 36-year-old pivotman had posted averages of 14.9 points, 7.8 rebounds and 2.3 assists over 65 games this season before missing Saturday's contest against Memphis.