The 25-year-old was placed on the 15-day disabled list at the end of May with a left forearm strain after giving up five hits and four runs (two earned) over a pair of rough outings.
"[He had a] good inning last night. I think the velocity was around 91-92 [mph], which I think will increase with confidence as he goes," manager Terry Francona said. "It was a good, easy inning."
Morales will rest Sunday and then likely pitch another inning Monday before being reevaluated.
The Red Sox acquired Morales less than a month ago in a trade with the Rockies for cash and a player to be named later.
The left-hander has shown flashes of brilliance, averaging seven strikeouts per nine innings over his career. But Morales has also struggled with his control, managing to walk 5.2 batters for every nine frames -- an issue the Red Sox's coaching staff are trying to correct.
Boston will also try to improve Morales' splits against right-handed hitters, who are batting .275 off him in his career as opposed to left-handers, who hit just .188.