It is difficult to back up an observation about a center with many hard figures, but I think there has been a slight decline in Nick Mangold's play the last few years. Mangold is not a weakness. He is still a quality starter at center. He just is not playing at the level that made him an All Pro a few years back.
According to Pro Football Focus, Mangold was responsible for 15 quarterback disruptions. That was 11th least in the league, certainly above average but not in the top three range like he was during his All Pro years from 2009 to 2011. According to Football Outsiders, the Jets' offensive line was 15th in the league in producing yards up the middle. Mangold was certainly only one component of that. The situation at left guard did not help, but I think Nick's decline was not only about that from watching things over.
At the risk of sounding like a heretic, I think it might be time for the Jets to start grooming a replacement. For 2014 Nick should surely remain the starting center. He is still a quality starter. Perhaps he can find that All Pro form again. In the long term, though, $8 million and up is a ton of cap space to dedicate to a center unless it is one of the top two or three centers in the league. Nick was there at one point. He hasn't been as much recently. Hopefully he can get there, but this feels like a good time to find a replacement and develop him.