Morales, 25, is 0-1 with a 3.86 ERA in 14 appearances this season.
Morales has also started 15 games in his big league career, but none since 2009. He was a starter throughout his minor league career until 2010, when he made 24 appearances in Triple-A as a reliever and another 15 relief appearances with Colorado. The Red Sox are currently desperate for starters, with John Lackey and Daisuke Matsuzaka on the disabled list. The team also signed Kevin Millwood to a minor-league contract on Thursday.
Morales struck out 11 batters in 14 innings this season, while walking eight.
Boston designated Hideki Okajima for assignment to make room for Morales.