Baas, the 33rd overall pick in the 2005 draft, was slated to start at right guard for the 49ers this season after starting nine games at the position last year. He will undergo surgery Friday after injuring himself while lifting weights last week.
Baas will miss at least part of training camp, likely including the preseason schedule in August. After a slow start to his pro career due to several injuries, the former Michigan center was expected to be a key part of the 49ers' offensive line this season following right guard Justin Smiley's departure for the Miami Dolphins.
The 49ers also don't know whether left guard Larry Allen will return for his 15th NFL season.
