The San Francisco Giants, already faltering in last place in the National League West, doubled-down on misfortune before starting a three-game series against the Rockies at Coors Field. They lost ace pitcher Madison Bumgarner for a significant amount of time after he crashed a dirt bike on a day off in Colorado, manager Bruce Bochy said Friday.

Bumgarner, a four-time all-star and the 2014 World Series MVP, suffered a sprained AC joint in his left shoulder and bruised ribs. He was placed on the disabled list, with no timetable for his return. He could be sidelined as long as two months, according to a source cited by Fox reporter Ken Rosenthal.

“Here’s a young guy, like a lot of us, we think we’re invincible,” Bochy said of Bumgarner at Coors Field. “He was just having some fun. He hit a slippery spot and went down.”

Bumgarner is on the DL for the first time in his career. A workhorse of dominating consistency, the 6-foot-5 left-hander has won 15 or more games in four of the past five seasons, with a sub-3.00 ERA in each of the past four.

The Giants, 6-10 entering Friday, were in the NL West cellar after losing a four-game series to the Rockies last weekend in San Francisco, then splitting a two-game set in Kansas City against the Royals. Bumgarner, despite pitching a complete game and totaling 28 strikeouts against just four walks in four games this season, is 0-3 with a 3.00 ERA.

He was injured Thursday after the Giants flew to Denver from Kansas City. After crashing, Bumgarner drove himself back to the team’s downtown hotel, then was taken to a hospital for examinations, including X-rays, MRIs and CT scans.

On Friday, he remained at the Giants’ hotel, where Bochy visited him before coming to Coors Field.

“If I was there, sure, I wouldn’t have let him on that bike,” Bochy said. “I’m sure, looking at it now, Madison wished he wouldn’t have gotten on it. He’s very remorseful. Unfortunately, an accident happened. Our focus is, let’s get him healthy. It’s happened. Let’s get him back on the mound. And hopefully it’s a lesson learned. You have to be careful with some of these activities you do.”

Dave Righetti, the Giants’ pitching coach, said days off often can be troublesome for baseball players, sucking them into letting loose.

“Off days are the freaking worst — for all of us, for years,” Righetti said. “That’s part of the game. We don’t get many, and when we do guys tend to want to go do those things that they’re missing out on.”

Bumgarner gave up only one run in seven innings Wednesday against the Royals, but the Giants lost 2-0. Last week, the Rockies knocked Bumgarner for three runs on six hits in six innings and beat the Giants 3-1. He wasn’t scheduled to pitch this weekend in Denver.

“Wow. Just, wow,” Rockies ace Jon Gray, who’s on the DL because of a broken foot, said Friday when he was told about Bumgarner’s injury.

The Giants on Friday welcomed back Bochy after a minor heart procedure kept him hospitalized overnight in San Diego while the team played in Kansas City. Also, all-star catcher Buster Posey missed a week while on the concussion DL. The Giants have won only one series this year and were four games behind the first-place Rockies before Friday’s series opener.

“You have to be grateful he’s not hurt worse than he is,” Posey said of Bumgarner. “You have to look at it in that light. I don’t know the timetable; I’m not sure anybody does. Just thankful he’s not hurt any worse.”