Coach Mike Singletary said Tuesday that Heitmann wasn't recovering quickly enough, and the team didn't want to push him beyond what he could do to get back on the field.
"We were telling him it was a week-to-week thing, but it's not making progress as fast as we would like or as fast as he would like," Singletary said. "After a while you kind of begin to press it a bit and [he] just didn't get to do that. We wanted to make sure we did what was in the best interest of Eric as well as the team and go ahead and put him on IR so he cannot try to speed up and do something that we don't need him to do right now, and at the same time try and get another guy to help us on special teams."
Singletary also said Tuesday he is planning to start Troy Smith at quarterback for a second straight game against the St. Louis Rams on Nov. 14 after the bye week. Regular starter Alex Smith separated his left shoulder in a 23-20 loss at Carolina on Oct. 24.
Smith could need two to three more weeks to recover.
The 30-year-old Heitmann was listed out with a neck injury, though he hadn't played all season after breaking the fibula in his left leg early in training camp Aug. 9. He was expected to miss up to two months and returned to practice Sept. 29. David Baas took his place on the starting offensive line.
Heitmann, a ninth-year pro out of Stanford, was going to miss at least another four weeks with a neck injury that also happened in training camp.
Before his injuries this year, Heitmann had started every game at center each of the past three seasons. He played 14 games there in 2006 before breaking his right leg, needing surgery and missing the final two games.
Replacing someone as essential as the guy who snaps the ball to the quarterback was a tough task. Baas hadn't played center regularly since his senior season at Michigan, but played one game there at the end of the '06 campaign in Heitmann's place.
"It's been exceptional," Singletary said of Baas' work filling in. "I remember talking to him during training camp when Eric initially was injured and just trying to get a feel for where he was in having not played the position in a while. And then being able to jump in there and do the job, I thought not only did he step up and do the job, but he's made great strides."
Heitmann tried to help in any way to make the transition smoother for Baas, even taking notes for his teammates and attending offensive line meetings. He will be forced to take on such a role again now.
Gibson was inactive for the Steelers' first six games before they cut him Sunday. A fourth-round draft pick this year out of Ohio State, selected 116th overall, the 23-year-old Gibson finished his collegiate career with 82 tackles, 10 sacks, 25 tackles for loss, five forced fumbles, three fumble recoveries and an interception in 35 games.
He and 49ers offensive tackle Alex Boone were college teammates from 2006-08.