Oakland's first round pick in the 2011 draft, Gray made two relief appearances last month, striking out six batters in four scoreless innings. He was 10-7 with a 3.42 ERA in 20 starts at Triple-A.
Gray's start means an extra day of rest for right-hander A.J. Griffin, who has allowed a major league-worst 28 home runs this season, 10 of them in his past four starts.
Rosales, who is batting .191 with four homers and eight RBIs, was designated for assignment for the third time in 11 days.