SAN FRANCISCO -- The Giants confirmed Friday what had been widely expected about Madison Bumgarner: Their pitching ace has a Grade 2 sprain of the AC joint in his throwing shoulder, besides bruised ribs, that will prevent him from rejoining San Francisco's starting rotation at least until the July 10-12

SAN FRANCISCO -- The Giants confirmed Friday what had been widely expected about Madison Bumgarner : Their pitching ace has a Grade 2 sprain of the AC joint in his throwing shoulder, besides bruised ribs, that will prevent him from rejoining San Francisco's starting rotation at least until the July 10-12 All-Star break.

The Grade 2 designation means Bumgarner has a partial tear in his shoulder. Giants manager Bruce Bochy said the four-time All-Star, who hurt himself in a dirt-bike accident during a scheduled off-day outside of Denver on April 20, began his rehabilitation procedure Friday with team physical therapist Tony Reale.

Head athletic trainer Dave Groeschner said Bumgarner, 27, will require about two months of rehab before he resumes throwing a baseball.

Bumgarner likely will need three to four weeks of throwing -- in bullpen sessions, simulated games and Minor League injury rehab assignments -- before he's ready to appear in a Major League game.

Should Bumgarner adhere to that timetable, he'd likely regain adequate form around the time of the All-Star break.

Reflecting Bumgarner's stature, the Giants' medical staff canvassed several renowned sports physicians for second opinions, including James Andrews, the reigning guru of Tommy John elbow surgery. All agreed Bumgarner would not need surgery, Groeschner confirmed.

Bumgarner was 0-3 with a 3.00 ERA and 28 strikeouts in 27 innings and a complete game in four starts before being sidelined.