David Diehl's situation is very similar to Corey Webster's with one big difference.
Like Webster, Diehl has spent his entire career with the Giants, is on the wrong side of 30 and was a big part of their last two Super Bowl wins. He is also coming off a bad 2012 season where he missed three games due to injury and didn't play well in the 13 games he took the field.
He had a -6.8 PFF rating largely due to the fact that he was a turnstile in pass-blocking. In only 487 snaps, Diehl allowed 19 quarterback hurries, seven hits and four sacks.
As a point of comparison, the best pass-blocking right tackle in the league for the 2012 season, Gosder Cherilus, allowed the same number of hits and sacks as Diehl (with eight more hurries) but accumulated 1,229 snaps.
The difference between Diehl and Webster, though, is that the latter had his contract restructured while the former is still carrying around an atrocious $6.825 million cap hit. If the Giants had thanked Diehl for the memories and sent him packing, they would have saved roughly $4.5 million.
That means it's time to play the "who could have been the Giants starting right tackle for the 2013 season if they had cut Diehl" game.
Cherilus would have been one realistic option. He signed a five-year, $34 million deal with the Indianapolis Colts, but if the Giants had back loaded the contract, he likely would have counted for less than $4.5 million towards the 2013 cap.
Also, given the size of Cherilus' deal, Big Blue could have probably also signed Eric Winston, who remains a free agent after being cut by the Chiefs on March 7. Winston is a few years older then Cherilus and was not quite as good in 2012 but he is still three years younger and significantly better than Diehl (Winston had a 13.1 PFF rating last season and played in 15 games).
Restructuring Diehl's contract should have been the bare minimum in dealing with his bloated cap figure. Cutting him, however, would have been the best move and would probably have led to them securing a younger, better starting right tackle then they currently have on their roster.