"He's in until he's out," Melvin said.
Gray has made two relief appearances for the A's, one before the All-Star break, on July 10 in Pittsburgh, and another after, on July 19 against the Angels, and pitched two scoreless innings in each, allowing only one hit and striking out three in each outing.
The 25-year-old was optioned shortly after his second appearance, at which point he resumed starting duties with Triple-A Sacramento.
"His two outings were very impressive, and we felt regardless of how we used him," Melvin said, "it was a good idea to get him to the big leagues and get his feet wet before potentially this type of start for him. It kept our options open, and once he went back down he continued starting. He got his pitch count up to where we felt comfortable for him to go out there with no limitations."
Gray, the A's third-ranked propect by MLB.com, takes the rotation spot of lefty Tommy Milone, whose recent struggles led to a demotion to Sacramento last week. Milone made his River Cats debut on Thursday, allowing two runs on eight hits with one walk and seven strikeouts in five innings.
When asked if there was a plan for when Milone might rejoin the team, Melvin said, "Not at this moment."
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.