While I'm not ecstatic about Kosier, he has started for SF (when they were decent) and Detroit. When I did my statistical study of all starting O-Linemen in the league, Kosier finished 16th for LG's and Larry Allen finished 20th. My problem with Allen is that he became incredibly one dimensional last year and D-Linemen started to shoot gaps from different angles and he couldn't get a hold of them anymore. He was awful in his backside blocking. It also didn't help that he doesn't really fit the Parcells blocking scheme which prefers having mobile O-Linemen who can get out in space and pull. To Kosier's credit, I think he at least fits the scheme better than Allen does at this point.
Fabini only started partially last year, so I couldn't really use my stats on him.
However, in 2004 he finished as the 2nd best LT according to my stats, behind Walter Jones. He was pretty average in pass blocking, but he led the league in run blocking.
Now, I don't believe my stat analysis of O-Linemen is completely concrete, but I think it gives a decent ballpark figure of how O-Linemen are doing.
It's not a question of if a healthy Fabini is an upgrade over Big Rob, it's by how much.
Rich........