Barry Zito's loss of velocity is one of the great mysteries of sports. The lefty came up throwing a 93 mph fastball that eh combined with a devastating overhand curve and change-up to quickly establish himself as one fo the top pitchers in the game. He went 23-5 with a 2.75 ERA to win the Cy Young in 2002. He was only 23 years old at that time, and the future looked bright.
When the Giants signed him it was obvious to everyone that they had overpaid, but most still expected him to be a solid pitcher. 15 wins a year, an ERA of around 3.50, and 200 innings every season, was a reasonable expectation. But then his velocity disappeared. As I said, when he burst onto the scene in Oakland his fastball was consistently at 91-93 mph. After the 2006 season, when Brian Sabean and Peter Magowan lost their collective minds, he was still at 88-90 with the fastball, occasionally even touching 91.
Inexplicably, Zito started the 2007 season throwing his fastball at 86-88 mph. This allowed him to be somewhat effective, and kept the Giants in a lot of games, but they were a bad team, so he finished with a sub-.500 record. This season, Zito's fastball has been between 82-84 mph. This is not enough to keep the Giants in games. Zito has been rocked most of the season, even skipped a start a few weeks ago. His past few starts have been better, but now he has to pitch very differently.
The Curveball is still good, and the change-up is still very good, but Zito has to throw the fastball at least 85 mph to get enough of a differential between that and his off-speed pitches to fool hitters. The man is only 29 years old and does not seem to have an injury. There is no reason the velocity can't come back, at least to the point that he can once again be an effective pitcher. He will never be worth $126 million, but I would argue he doesn't have to be. With Cain, Sanchez, Lowry, Lincecum, Correia, and even Misch in the rotation, the Giants have a plethora of young starting pitchers. Zito has by all indications done a good job mentoring these young pitchers, and their positive performances appear to be further proof of this. All of these pitchers are young, a couple have signed long term deals, but most are not even eligible for arbitration yet.
If the Giants are smart, they can hold onto the youngsters, which will prevent Zito's contract from becoming a complete albatross.