Just as Ainsworth's career was looking very promising, it started falling prey to serious arm injuries in . He began the season in the Giants' starting rotation and made 11 starts for them; he went 5-4 with an ERA of 3.82 in 66 innings pitched. On June 3, 2003, he was placed on the disabled list with inflammation in his shoulder. He made a rehabilitation appearance with Fresno on June 15, but lasted just two innings; later in the month, doctors found that his shoulder blade was broken, an extremely unusual injury for a pitcher. On July 31, the trading deadline, the Giants traded him and Damian Moss to the Baltimore Orioles for Sidney Ponson. He was not activated from the disabled list until September 20, and he appeared in just three games for the Orioles in 2003, pitching 2 innings in relief and accumulating an 0-1 record with an 11.57 ERA.