49ers running back Isaac Guerendo tore a pectoral muscle about a month ago while lifting weights, coach Kyle Shanahan said.
Shanahan said Guerendo could be ready by the end of training camp.
The 49ers signed running backs Jordan Mims and Jermar Jefferson on Thursday to help fill in while Guerendo is out.
Guerendo has an injury history, with hamstring, groin, foot sprain, ankle and shoulder injuries since joining the 49ers as a fourth-round pick in 2024.
He did not play an offensive snap in 2025, but did see action on 173 special teams in 14 games.