Michael Ola and Brian de la Puente have been preparing for this since the start of Chicago Bears training camp, if not longer.

A year after going through the entire regular season without a starting lineup change on their offensive line, the Chicago Bears are looking at the possibility that two spots will be manned by reserves at San Francisco on Sunday night due to the ankle injuries to center Roberto Garza and left guard Matt Slauson.

Garza and Slauson missed practice Wednesday and have been called "day to day" but coach Marc Trestman.

"We'll see where they are tomorrow," Trestman said.

They aren't the only injury concerns, as starting wide receivers Brandon Marshall (ankle) and Alshon Jeffery (hamstring) also missed practice Wednesday. Yet the injuries to the two linemen appear more serious. The team brought guard-tackle Eben Britton back to the 53-man roster Tuesday after he'd been cut in preseason.

"The best way to handle these type of situations is never to get caught off guard," Ola said.

For Ola, the rookie and former Canadian Football League lineman, he literally was off guard. He played at right tackle throughout training camp and most of preseason because of a foot injury to starting right tackle Jordan Mills. Suddenly he's playing guard for Slauson.

"I've got a fortunate ability to be able to play multiple positions," he said.

"The biggest thing would just be to always stay ready so that you never have to get ready, and with the way this season has been going and with the way things happened in OTAs and in training camp, I've been thrust into position to really have to know this offense," he said. "It actually was a blessing in disguise because now I've gotten a lot of familiarity with the starters, being able to be comfortable, being able to have the guys who start trust me enough to know that if I come in there they don't have to be looking over their shoulders and all."

For de la Puente, the move into the starting lineup might be at a familiar position, but he spent the last four years with New Orleans and had gotten much of his preseason playing time at guard or with backups at center. Then again, he's playing in a similar offense under coordinator Aaron Kromer, his former line coach with the Saints.

"It definitely makes the transition easier," de la Puente said. "I'm familiar with the schemes. The offense, you know, obviously it's different. There was some learning to do."

Britton, who had failed to make the final cut, had been the only backup offensive lineman to play last year. Bringing him back could be further indication they expect to be without the starters a while, even if they're not saying it.

The situation at wide receiver appears less severe because Jeffery had been listed probable to return Sunday after his injury, and Marshall finished the game. Still, not having either made for a difficult practice Wednesday because the team had also cut backup Michael Spurlock Tuesday and backup Josh Morgan was available only on a limited basis Wednesday due to a groin injury suffered Sunday.

Santonio Holmes was their only entirely healthy wide receiver for practice, and didn't join the team until just before the third preseason game. Josh Bellamy, Rashad Ross and Greg Herd are the practice squad wide receivers.

"Certainly, there's guys there who haven't played, but they've practiced," Trestman said. "They know the system. We'll make sure we're in a position that allows them do their jobs and we'll go from there."

NOTES: Fullback Tony Fiammetta, who missed the opener due to a hamstring pull, remains out of practice. Third quarterback David Fales also is out with a shoulder injury which kept him from being considered for backup duty in the opener. ... Cornerback Charles Tillman was held out of practice due to a coach's decision, but is not injured.