SAN DIEGO -- The San Diego Chargers could be starting their fourth center this season against the St. Louis Rams.

Starting center Rich Ohrnberger missed his second straight practice with ankle and back issues Thursday. Ohrnberger is still wearing a walking boot after injuring his ankle against the Oakland Raiders last week, and has been dealing with a nagging back issue for most of the season.

Rookie Chris Watt, who has played center this season as an injury replacement during games, would get his first career start if Ohrnberger can't play Sunday.

Offensive coordinator Frank Reich said he feels comfortable with Watt starting at center if Ohrnberger can't go. Watt has been rotating with Johnnie Troutman at right guard. But if Watt starts at center, Troutman would play the entire time at guard.

"Here we are going into 11 games into his rookie year, and it's the old adage that he's no longer a rookie," Reich said about Watt. "He's proven himself. He's tough. He's smart. And I look at him like he's been here 10 years. You know that's not true, but there's a side of it of that's how it feels. And that's how it has to feel for him."

Along with Ohrnberger, defensive tackle Ryan Carrethers missed practice for a second day in a row with an elbow injury. Safety Jahleel Addae was listed as a limited participant Thursday after a practice as a full participant Wednesday.

Chargers head coach Mike McCoy would not say if Addae suffered a setback. Addae has missed two games and been out three weeks since suffering a concussion Oct. 23 against the Denver Broncos.

"Just like every player, we're going to take them day by day, and see how they feel from day to day," McCoy said. "And we'll let them work accordingly. That's what we did with Jahleel today."

Outside linebacker Dwight Freeney returned to practice after taking a rest day Wednesday. Philip Rivers (chest), nose tackle Sean Lissemore (quad) and cornerback Steve Williams (hamstring) were full participants.