Smith suffered the injury during Saturday's practice, which was held in front of over 25,000 fans at Levi's Stadium.
Rough blow: #49ers veteran LB Malcolm Smith is out for the season with a torn pec, sources say. He replaced Smith in the starting lineup Saturday as the weak-side inside linebacker during the full-team session.
"There's some stuff in his knee we're probably going to have to clean up", Shanahan said, via Eric Branch of SFGate.com. "We don't expect it to be anything long-term, but it's going to be a little bit of time".
Garnett has primarily played at left guard with the first-team unit during training camp.
More information regarding Smith's injury and replacement is expected to come out in the coming days and a decision should be made relatively soon. Though not pretty, the 49ers starting offensive line will likely consist of Joe Staley, Zane Beadles, Jeremy Zuttah, Brandon Fusco, and Trent Brown.
He appeared in 15 games for San Francisco last season as a rookie, starting 11 of them at right guard.
But the injury also opens up the door for Foster to see an increased role.
Foster was projected to be a top-15 pick in the 2017 NFL Draft, but questions regarding a shoulder injury led to him falling to the bottom of the first-round. But if Smith's injury is serious, the team isn't really left with any other choice. There hasn't been much news about how Armstrong has performed so far in training camp, so one has to wonder if those two games were an anomaly for a historically erratic player, both on and off the field.