2005 saw the young prospect starting the season off in Wilmington, Boston's High-A ball affiliate. After posting a 6–1 record with a 2.40 ERA, receiving a selection for the All-Star Futures Game, and making the Carolina League All-Star team, he was promoted to the Double-A Portland Sea Dogs. Sánchez got off to a hot start there, but tired toward the end of the season, creating some concern about his durability. Still, Sánchez put up a 3.45 ERA over 11 starts in his first year in Double-A; at 21 years old, he was still one of the youngest players in his league.