The San Francisco 49ers paid a heavy price for Sunday's blowout 31-13 win over the New York Jets.
Nick Mullens, who came in against New York, will start if Garoppolo is sidelined.
Injuries have decimated San Francisco to start the season.
Running back Raheem Mostert also left the victory over the Jets due to a knee injury. The 49ers believe Mostert suffered a mild sprain of the MCL and his return timetable now depends on the MRI, according to Schefter.
Bosa's likely season-ending injury is an enormous loss for a 49ers defensive line that helped carry the team to the Super Bowl last season.
The former Ohio State standout was named the Defensive Rookie of the Year after a nine-sack debut campaign in 2019.