The 49ers placed Webster (hamstring) on injured reserve Tuesday, Nick Wagoner of ESPN.com reports.
Coach Kyle Shanahan said that Webster suffered a "pretty severe hamstring injury" during Sunday's win over the Rams, according to Eric Branch of the San Francisco Chronicle. In turn, he could be in danger of missing the rest of the season, although he'll be eligible to return for a Week 16 matchup against the Cardinals. Webster's absence will mainly affect the 49ers' special-teams coverage.