An MRI this week revealed the sprained medial collateral ligament, and Iupati isn't expected to need surgery, the person said, requesting anonymity because the 49ers didn't reveal specifics on what happened to Iupati.
He walked to the sideline with assistance in the third quarter of a 23-20 loss at the Superdome and then was carted off to the locker room. The injury happened when running back Frank Gore got tackled into the back of Iupati's left leg.
The 26-year-old Iupati, a staple in San Francisco's veteran, much-improved O-line, has started every game since the 49ers selected him 17th in the 2010 NFL draft out of Idaho. That run would end if he isn't able to play Monday night at Washington.
"I don't know the timetable or how much time he might miss," the person said. "I'm not sure Mike knows. They're still evaluating him."
Iupati wasn't in the locker room during the portion open to reporters Wednesday, when the reigning NFC champion Niners (6-4) canceled a scheduled walk-through because of a downpour and moved their preparations for the Redskins indoors.
Coach Jim Harbaugh will next address the media Thursday. Adam Snyder, who didn't speak to reporters, would be the likely substitute to replace Iupati.
"Mike's a great player, a Pro Bowl player," running back Frank Gore said Wednesday. "He helps out our running game a lot, but Snyder is a vet player. He's been playing since he was a rookie. He'll be fine. But we will miss Mike. I think Snyder can come in and play."
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