Though his age and increasingly poor speed earned him a reputation as a defensive liability in the outfield, he still possessed a strong throwing arm, and was considered a perfectly capable fielder at first. In 2008, Stairs initially platooned in left field with Shannon Stewart; however, upon the club's release of Frank Thomas, Stairs became the everyday DH for the ball club. Stairs was designated for assignment on August 28, 2008.
On August 30, 2008, Stairs was traded to the Philadelphia Phillies for Fabio Castro.
Stairs hit his first career postseason home run on October 13, 2008 in Game 4 of the 2008 National League Championship Series against the Los Angeles Dodgers off Jonathan Broxton, allowing the Phillies to take the lead and win the game. In the 2009 season, he once again made it to the World Series.
He won the first World Series ring of his 16-year career on October 29, 2008, when the Phillies won the series against the Rays, 4 games to 1. On April 12, 2009, Stairs' game-winning home run against the Colorado Rockies was the last home run called by legendary broadcaster Harry Kalas, who died unexpectedly less than 24 hours later.