Mustapha (chest) has been ruled out ahead of Thursday night's matchup against the Rams, Eric Branch of the San Francisco Chronicle reports.
Mustapha likely sustained a chest injury in the 49ers' blowout win over the Bears in Week 14, and after closing the week of practice with a DNP on Wednesday, he'll be sidelined for Thursday night's divisional matchup. Expect Talanoa Hufanga, who just returned from injured reserve last week, to start opposite Ji'Ayir Brown as part of San Francisco's top safety duo in Week 15.