Another concussion clouds the future of Lions center Travis Swanson

Detroit Lions center Travis Swanson has had his season end because of a concussion for the second straight year.

He was officially placed on injured reserve Friday, weeks after concussion symptoms first surfaced after Swanson played every snap in Detroit's win Dec. 10 against Tampa Bay. Swanson missed the next two games and then returned to practice as a full participant Wednesday.

He was downgraded to no practice Thursday and then placed on injured reserve Friday.

Last season, Swanson played every snap in the Lions' win against the Saints on Dec. 4. The following week, the Lions placed him in concussion protocol and he missed the rest of the season.

This season, the Lions have had three offensive linemen -- Swanson, T.J. Lang and Don Barclay -- play a game, finish the game and then have concussion symptoms in the days that followed. Lions coach Jim Caldwell was asked about this Thursday and said he believes in the league's protocol for dealing with concussions.

"Everybody's a little bit different. I mean, Nevin (Lawson) was in the ball game. His manifested itself rather quickly. And then you find others that are delayed somewhat," Caldwell said. "I mean, medically that's what they say. They can manifest themselves 48 to whatever 72 hours afterwards. And it just so happened that we've had a few guys that have done it. But I think our doctors do a great job with it.

"Once it's identified, they put them through the protocol, and they do what the League has advised us to do. And the other part of it is that I just think the League's done a number of different things to help in those areas. All kinds of protocol, all types of changes, every year it's being evaluated, and so it's something that's being constantly reviewed."

Caldwell said Thursday that he wasn't sure how often players on the Lions have self-reported concussion symptoms versus being taken out after being evaluated for one.

Swanson is in the final year of his rookie contract with the Lions and is to become a free agent in March.

To replace Swanson on the roster, the Lions signed receiver Jace Billingsley.