According to fangraphs.com, Matsuzaka has lost nearly 4 MPH on his fastball, dropping it to below 90 MPH, which, in a lot of cases, is a warning flag to pitchers that they are on the decline and cannot get batters out. When Dice-K entered the MLB, he was able to throw a heater up to 97.4 MPH. In his first start for the Mets against the Tigers, Matsuzaka's fastball topped out at 91 MPH. Dice-K has lost something in his MPH, and hasn't been able to change his style of pitching.
Also according to fangraphs.com, Matsuzaka pitches way too much in the white on white, when he should be living on the black, especially due to the loss of velocity in his fastball. When Matsuzaka entered the MLB in 2007, he could throw hard, so although he pitched in the hitters' "sweet spot," he was able to escape, due to his high velocity. But, now, pitching with his current velocity, hitters are able to "tee off" of Matsuzaka's pitching. The numbers show it too.
In 2007, Matsuzaka had a 4.40 ERA and threw an average 92 MPH fastball (97 high).
In 2008, Matsuzaka had a 2.90 ERA and threw an average 92 MPH fastball (96 high).
In 2009, Matsuzaka had a 5.76 ERA and threw an average 91 MPH fastball (95 high).
In 2010, Matsuzaka had a 4.69 ERA and threw an average 92 MPH fastball (95 high).
In 2011, Matsuzaka had a 5.30 ERA and threw an average 90 MPH fastball (93 high).
In 2012, Matsuzaka had an 8.28 ERA and threw an average 90 MPH fastball (95 high).
In 2013, Matsuzaka has a 9.00 ERA and throws an average 88 MPH fastball (91 high).
Although it seems like his velocity numbers are the same, every MPH lost is another split second for the hitters to hit. Dice-K hasn't been able to evolve his game, so the hitters can practically guess what pitch is coming in a certain count. Matsuzaka should watch some film of Tom Glavine, a 300 game winner who only could throw in the high 80s. Dice-K needs to learn how to live on the black, paint corners, and be able to attack hitters with something besides a fastball.
The question is, can Matsuzaka become an effective pitcher again? I believe that if he is able to detect a problem in his mindset when pitching. Matsuzaka still has MLB-caliber stuff, and can have a successful comeback. The Mets took a low risk signing in Matsuzaka, which in my opinion, could be a great bargain, if he can replenish his career. The problem is that he hasn't been able to do that for a long time.