Logan Mankins, former Patriot
I don't know enough about Pro Football Focus' statistics and tracking to say that this is entirely correct, but I think it's pretty clear that the Patriots agree with the basic premise of this piece, that there's a significant disconnect between Mankins' cost and Mankins' value.
2. Logan Mankins, Left Guard(Emphasis mine.)
Mankins signed a contract extension three seasons ago that made him the highest-paid guard at the time. His contract remains the second-highest in average per year, but his play has taken a dip over the last couple seasons largely as he plays through injuries. His performance has still been positive (+12.0 in 2013), but nowhere near elite status like he is being paid. He set to count over $10 million against the cap again in each of the next two years. Unless Mankins, who turns age 32 tomorrow, is able to regain some of his elite form, he will likely remain among the most overpaid on the Patriots’ roster for a third straight season.
2013 Cap Hit: $10m
2013 Jahnke Value Model: $4.2m
Value Differential: -$5.8m
I am definitely not of the "Bill can do no wrong camp." But I recognize that there are situations where I've got nowhere near enough information to make an informed criticism, and this is one of those cases. One of the reasons that the Patriots have been consistently good for the last 14 years is that Belichick is an economist. I don't see much, if any, evidence that he's any better at talent evaluation and prediction than anyone else, but he's very good and disciplined at allocating his resources.
This move may be a mistake, because it may be that Mankins was enough better than any of the replacement alternatives to justify the cost of keeping him on the roster. But it's not inconceivable that the replacement will weaken the run blocking slightly while improving the pass blocking, and on the whole, the offense won't suffer. Since I can imagine a scenario where this improves the team as a whole, and don't have enough information to argue convincingly against it, I am left without a good reason to condemn it. Not exactly, "in Bill we trust;" more like "Bill definitely knows more than I do and I hope he's right..."