When Barry Zito was signed to a seven-year, $126 million contact prior to the 2007 season, it was assumed that he was going to be the Giants' ace of the future. Unfortunately, that was not a fair standard to hold against him.
For starters, at the time of the contract, Zito was about to turn 29 years old and had already pitched seven seasons in the majors with the Oakland Athletics (he also pitched over 210 innings each season in that time span, with the exception of 2000, his rookie year).
So, when Zito didn't live up to expectations, it was natural for Giants fans to bash and criticize him for not "earning" his contract.
However, with Tim Lincecum assuming the mantle as the team's ace, the results have been much different. When Lincecum was made the team's Opening Day starter in 2009, Zito went on to have his best season as a Giant, posting a 1.35 WHIP, a 4.03 ERA and a 4.31 fielding-independent pitching metric (his best FIP since 2003, when it was 4.05).
With Lincecum the front-line starter again, Zito has continued to do well this year in 2010.
Thus, with Lincecum the main man in the rotation, Zito isn't forced to be the "guy" anymore in the rotation and isn't required to shoulder the load and hopes of this Giants team. He is able to pitch without much pressure (which he did in Oakland because he pitched along with Tim Hudson and Mark Mulder), which caters to his laid-back attitude and approach.
Would Zito be the pitcher he is today without Lincecum? Maybe, but I would highly doubt it. Zito isn't a No. 1 starter anymore. However, as a middle-of-the-rotation guy, he is still very good.