Nalen tore the biceps muscle in his right arm.
"Anytime you lose a great player it hurts you," coach Mike Shanahan said of Nalen's injury.
Nalen tore the muscle in the first quarter Sunday, Shanahan said, but played the rest of the game despite the injury. Nalen is signed through 2008, and Shanahan said he thinks Nalen will return. The recovery time after surgery is four months, so he could be able to play next year.
This year, the Broncos don't have many experienced options. Starting left guard Chris Myers played center in college, and he likely will move over. Another candidate is Greg Eslinger, who is on the practice squad. Denver could also look to sign a center, but the available crop is thin five games into the season.
Ben Hamilton would have been a candidate to take over at center. He played center in 2002 when Nalen suffered a season-ending knee injury, but Hamilton is on injured reserve because of post-concussion symptoms.
--The Broncos re-signed receiver Brian Clark to return kickoffs after cutting Domenik Hixon, who didn't have many long returns in the first four games. But Clark had a big mistake Sunday, fumbling in the first quarter. The fumble was returned by San Diego for a touchdown and a 14-0 lead, which started a blowout.
"I have to put it on myself," Clark said. "The guy put his helmet on the ball, and I wasn't technically sound as far as holding the ball."
The Broncos will re-evaluate their return game over the bye week.
--Broncos coach Mike Shanahan said defensive end Simeon Rice isn't healthy yet, which is why he was inactive against San Diego. Rice had surgery on his left shoulder. Rice said he is fine, but Shanahan doesn't see it.
"You watch him play and you watch him practice, and that shoulder is probably about 80 percent," Shanahan said. "When he is healthy, we think he brings a lot to the table, but he isn't right now."
--TE Nate Jackson tore his groin and is out the rest of the season. Jackson had a good training camp and took over as the second tight end, behind Daniel Graham. Tony Scheffler has been recovering from a broken foot, and he will take Jackson's role in the offense.
"He's gotten some practice time and playing time over the last couple of weeks, so he should be ready to go," Shanahan said of Scheffler.
PLAYER PERSONNEL NOTES
--LG Chris Myers is probably the top candidate move to center to replace Tom Nalen. Myers is a versatile lineman who entered the starting lineup because of Ben Hamilton's concussion.
--CB Champ Bailey has a quadriceps injury, but tests were negative. Broncos coach Mike Shanahan said Bailey probably wouldn't have been able to play if the Broncos had a game this week but should be ready after the bye.
--WR Javon Walker should be ready after the bye, after missing two games with a knee injury. But coach Mike Shanahan refused to speculate on whether Walker would be back after the bye, remaining cautious until Walker practices.
--G Chris Kuper could get a starting job. Kuper was slated to start at right guard most of the offseason, but he lost the job to Montrae Holland. If Chris Myers moves from guard to center, Kuper is the obvious candidate to replace Myers at left guard.
--RB Andre Hall could get a look at kickoff returner. The Broncos haven't had much success at kickoff return this season. Domenik Hixon struggled for four games, then was cut. Brian Clark had a huge fumble in last week's loss.
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