San Francisco Giants ace Madison Bumgarner has been diagnosed with a Grade 2 sprain of his throwing shoulder and is expected to miss at least three months, the team announced Friday.Bumgarner, who injured his shoulder in a dirt bike accident earlier this week, tentatively is scheduled to return around the All-Star break.Dr. Kenneth Akizuki, the team's orthopedist, led the medical process although outside specialists were consulted, csnbayarea.com reported.Dr. Tim McAdams, the team doctor for the NFL's San Francisco 49ers, saw Bumgarner and his test results were sent to Dr. James Andrews and Dr. Neal ElAttrache. All of the doctors agreed that Bumgarner does not need surgery."That's nice to hear," Giants trainer Dave Groeschner told csnbayarea.com. "It's nice when everyone is in agreement. Now we'll put a plan in place and work on the rehab and get him back. Bum is a very conscientious person. We feel good about the fact that if we're going to ask him to do something (to rehab), he's going to do it."Bumgarner sustained bruised ribs and the sprained AC joint in his pitching shoulder during last Thursday's off day in Colorado. The former World Series MVP will wear a sling on his pitching arm for at least another week.Bumgarner never previously was on the disabled list during his eight-plus major league seasons.The 27-year-old said Monday he regretted the decision to ride trail bikes in the mountains outside of Denver with two family members.Bumgarner is 0-3 with a 3.00 ERA this season.