Andrew Baggarly of the Bay Area News Group and Alex Pavlovic of NBC Sports Bay Area reported the news. Bumgarner has been diagnosed with a partial tear to the AC joint in his throwing shoulder but will not require surgery.
The 27-year-old lefty was 0-3 with a 3.00 ERA and 1.07 WHIP after his first four starts. He last pitched April 19 in a loss to the Kansas City Royals.
The injury was the result of a crash on a dirt bike he suffered during the Giants' off day.
"I wish I had some kind of cool story for you that it was some kind of crazy wreck, but it really wasn't anything spectacular, just super unfortunate. I was actually being pretty safe the whole time; it was just a freak deal. It happened right at the end. That's really it," Bumgarner told reporters.
The Giants, currently sitting in last in the NL West, will have to make do with their makeshift rotation without their ace for now. Johnny Cueto, Jeff Samardzija and Matt Moore make for a fine top three, but this was a San Francisco team built on pitching.
With the offense struggling mightily to start the season, this crash couldn't have come at a worse time.