Injuries mounting for O-line

The 49ers are running out of right tackles.

On Monday, head coach Kyle Shanahan announced that Erik Magnuson would be placed on injured reserve with a foot sprain he suffered in a 24-13 loss to the Seahawks on Sunday. Magnuson, an undrafted rookie, was subbing for starter Trent Brown (shoulder), who couldn’t play because of discomfort.

Shanahan said Brown remained sore Monday and was unsure if he would be able to play against the Bears on Sunday.

“We’ll see how it goes throughout the week,” Shanahan said.

If Brown can’t play, the 49ers likely would turn to undrafted rookie Darrell Williams Jr., who would be their fourth starting right tackle in their past five games. On Oct. 29, Garry Gilliam, who was replacing Brown, was lost for the season with a knee injury.

On Sunday, Magnuson hurt his foot early in the second quarter.

“I was pretty amazed,” Shanahan said. “Got a lot of respect for that guy for staying out there and battling through that, especially with how short we were at tackles.”

Magnuson might have a future with the 49ers, given his versatility and performance in two starts at right tackle this season. He was practicing at guard and center before injuries forced him to play outside. On Sunday, he faced Pro Bowl defensive end Michael Bennett, who had a sack and two quarterback hits.

“The game is not too big for him,” Shanahan said. “He’s not scared to fail. You get an undrafted rookie guard out there starting at right tackle for you going against Michael Bennett playing with a foot sprain for three quarters. … He doesn’t flinch, doesn’t hesitate, just keeps playing, doesn’t say anything. That’s a guy I’d love to play with.”

Shanahan said the 49ers would bring in tackles for tryouts Tuesday. They also could promote tackle Pace Murphy from the practice squad. More injury updates: Running back Raheem Mostert also will be placed on injured reserve with a knee injury that Shanahan said involved his MCL.

Mostert, who had stood out as a special-teams gunner, could be replaced on the roster by practice-squad running back Jeremy McNichols ,a rookie fifth-round pick of the Buccaneers. Tampa Bay waived McNichols on Sept. 2. The 49ers also will bring in running backs for tryouts Tuesday.

Also, Shanahan said rookie free safety Adrian Colbert (thumb) would be limited in practice Wednesday and his status is in question for Sunday. Eric Branch is a San Francisco Chronicle staff writer.