Brennan led all 2008 NFL rookies in touchdown passes, passing yards, and quarterback rating through the preseason. He finished the 2008 preseason with a strong performance, going 36-for-53, for 411 yards and three touchdowns and with no interceptions. For the preseason, his completion percentage was 67.9% and his quarterback rating was 109.9. He also carried the ball twice for 11 yards. He was placed on injured reserve on September 4, 2009, thus ending his 2009 season, due to a torn hamstring and a hip injury. The hip injury was similar to one he suffered while in college at Hawaii and required surgery.