Coach Kyle Shanahan said Monday that Nzeocha (quadriceps) likely will be placed on injured reserve, Nick Wagoner of ESPN.com reports.
Nzeocha suffered the injury late in Sunday's win over the Giants and didn't return. It appears he's dealing with a serious issue, though, so he'll likely spend at least three weeks on the sidelines as he recovers. Nzeocha rarely plays on defense, but his absence will hurt the team's linebacker depth and special-teams units.