Mitchell was injured during Sunday's 13-0 victory over the New Orleans Saints. It is Mitchell's second knee injury of the season.
"He's pretty disappointed. Everyone knows how good Elijah's been, how hard he's worked to get back from his last one and how good he's been playing since he came back," Shanahan said during a conference call with reporters. "To be right back there, I know he was real down on it. He's had some real bad luck. He got rolled up on and it was a bad position for him to be in."
Mitchell sprained his right MCL in Week 1 and missed the following seven games before returning in Week 10 against the Los Angeles Chargers.