I am going to group guard together because I don't think there is enough difference to warrant two posts.
Starter: ...Chris Spencer?
Age: Turned 28 March 28.
Health: Average to below average.
Contract: One year remaining.
Performance: Incomplete at guard, average to good at center.
Backup: Mike Gibson
Age: Turns 25 November 18.
Health: Incomplete, but spent 2008 on IR.
Depth: Mansfield Wrotto
Age: Turns 26 October 12.
Contract: One year remaining, plus RFA.
Depth: Steve Vallos
Age: Turns 27 December 28.
Contract: Two years remaining, plus RFA.
Performance: Mostly terrible.
Position need: Not as bad as I first thought, but rumors are flying that Spencer is on his way out. Gibson and Wrotto are the wildcards. Wrotto is talented but has never made a dent. Gibson is gritty but significantly less talented. I am not so sure any of the above fit what Gibbs wants. Vallos was achingly bad his rookie season, but played better in 2009. Maybe Gibbs can find the animal in the man and turn him into a serviceable regular.
Seattle either needs to add one starter or two. Assuming Spencer is out, it probably needs two. One might be produced internally, but none of the above candidates are favorites that I know of. That is bad, again. Gibson, Wrotto and Vallos seem like depth, and if a starter emerges, cool, but right now Seattle is relying on them. I am not sure any of the three will be on the roster by September, but I am sure a starter capable player or two must be added.