The five-time Pro Bowler was injured in the first quarter of a 41-3 loss to San Diego on Sunday, but kept playing.
"We couldn't even run inside running plays, had to run outside running plays," coach Mike Shanahan said Monday. "He didn't have any strength in there. It kind of gives you an idea of what he's made of."
The Broncos also lost tight end Nate Jackson for the season with a torn groin. Cornerback Champ Bailey, who injured his left quadriceps against the Chargers, should be ready for the Pittsburgh game on Oct. 21. The Broncos have a bye this week.
"If we were playing this week, I don't think he'd be able to play," Shanahan said. "He should be fine for Pittsburgh."
Shanahan said he didn't want to discuss who would start in Nalen's place, saying filling the void will be difficult. Chris Myers is listed as the backup on the Broncos' depth chart.
"Tom, I believe, is the best center to ever play the game," Shanahan said of the 14-year veteran. "That's what I believe."
Nalen is expected to have surgery within a week and will be out for close to four months. Shanahan didn't think the injury was career ending. "He should be fine and ready to go next season," Shanahan said.
Nalen is the second starting offensive lineman to go down. The team lost guard Ben Hamilton to a concussion in training camp. He was placed on season-ending injured reserve on Sept. 28.
Nalen hasn't missed a start since tearing a ligament in his left knee against Kansas City on Oct. 20, 2002. He had surgery to repair the knee and was back 10 months later in time to start the 2003 season, during which he earned his fifth Pro Bowl selection.