John Shea of the San Francisco Chronicle provided the updated timetable Friday night. The ace was originally projected to complete his recovery around the All-Star break in mid-July.
Bumgarner suffered the injury after an accident while riding a dirt bike in late April.
Carl Steward of the Mercury News passed along comments from the 27-year-old starter, who said he'd walked away from more serious wrecks unharmed when he was younger and wasn't even sure how this incident led to a long-term injury.
"It's terrible," Bumgarner said. "Obviously, it was not my intentions when we set out to enjoy the off day. I realize that's definitely not the most responsible decision I've made. It sucks not being able to be out here with my guys."
Pat Graham of the Associated Press noted Giants manager Bruce Bochy hoped the left-hander would make a full recovery and learn a lesson from the ordeal.
"Here's a young guy, like a lot of us, you think you're invincible," he said. "Our focus, it's on, 'Let's get him healthy.' It's happened. Let's get him well and get him back on the mound. Hopefully, it's a lesson learned ... You've got to be careful in some of these activities we do."
The 2014 World Series MVP posted a 3.00 ERA and 1.07 WHIP with 28 strikeouts in 27 innings across four starts before the injury.
Johnny Cueto, Matt Moore, Jeff Samardzija and Matt Cain will continue to lead a deep Giants rotation in Bumgarner's absence. San Francisco has already fallen 10 games off the pace of the National League West-leading Colorado Rockies in the 2017 season's early stages, however.