In the 2008 season, Pérez was inconsistent, posting a record of 10–7 with a 4.22 ERA. He was also prone to big innings striking at any moment. An example was in a start against the San Francisco Giants, he went of an inning allowing 5 hits and 6 runs, all of them earned. An example of his dominance was his start against the New York Yankees on June 29. Pérez went 7 innings allowing just one run on a home run by Wilson Betemit, striking out a season-high 8 batters. After the firing of manager Willie Randolph, Pérez pitched better. At the request of Pedro Martínez and pitching coach Dan Warthen, he changed his delivery to the plate. Instead of letting his head hang down when he made his delivery, he made a bowing motion. He led the majors in walks, with 105, and was 10th in the NL in wild pitches, with 9. His 17 no decisions were the most among MLB starting pitchers in 2008.