Coach Jim Zorn said Monday the veteran right guard has a complete tear of the right triceps off the bone, suffered in the first half of Sunday's 9-7 win over the St. Louis Rams. Thomas will have surgery this week and will be placed on injured reserve.
"We're really disappointed," Zorn said. "He is very disappointed."
Zorn said he isn't sure who will start at right guard next week against the Detroit Lions. One option is Will Montgomery, who last started a game in 2007 and filled in after Thomas left Sunday. Another is second-year player Chad Rinehart, who has never played in an NFL game.
Zorn said he's also thought about bringing back veteran Pete Kendall, who started at left guard last year but was not re-signed in the offseason. Even so, the coach said it's unlikely a new player could start against the Lions.
"I don't know if we can go outside the organization right now and have a guy ready for Detroit," Zorn said.
Thomas' injury weakens an offensive line that was fragile from the start of training camp. Thomas already had a long history of injuries, and left tackle Chris Samuels has to rest his aching knees from time to time. None of the five backups who made the 53-man roster played a single snap in the NFL last year.
Thomas tore his left triceps in 2007 and played in only three games. He also had knee and neck surgeries after last season. The neck operation was considered potentially career-threatening, while the knee surgery had coaches during training camp concerned whether he could play a 16-game schedule.
Now there is genuine concern whether the 33-year-old Thomas will play again at all.
"I am just very disappointed right now," Thomas said in a statement released by the team. "It hurts because I want to be out there with my teammates. I worked really hard in the offseason and was looking forward to this year. I will have the surgery this week and focus my attention on rehabbing."