To fill his spot on the roster, Boston added newly acquired right-handed reliever Ryan Cook.
Manager John Farrell said Sunday the Red Sox initially thought of just pushing Porcello's next outing back.
''We feel like it's a short-term thing,'' Farrell added.
The 26-year-old Porcello was expected to be a top-of-the rotation starter, but he is 5-11 with a 5.81 ERA in 20 starts.
The club signed him to a four-year, $82.5 million contract extension in April.
Boston acquired Cook from Oakland at the non-waiver trade deadline Friday in a deal for a player to be named or cash. He was pitching for the Athletics' Triple-A affiliate.