Giants manager Bruce Bochy discussed his decision with both pitchers he considered for the honor: Bumgarner and right-hander Matt Cain, last year's Opening Day starter.
"I'd be comfortable with either one going the first game," Bochy said.
But Bochy wanted to reward Bumgarner, who ranked third among NL leaders in lowest opponents' batting average (.203), fifth in ERA and tied for seventh in strikeouts (199). Bumgarner also topped the Giants in victories, innings (201 1/3), strikeouts, ERA and opponents' batting average.
Cain understood Bochy's choice, which was divulged after Bumgarner left Scottsdale Stadium.
"I'm happy for him," Cain said. "I'm pumped for him. I'm excited to watch him go out there, start Opening Day and get the season started right for us. It's such an honor to be able to go out there and pitch the first game. He's more than deserving."
Bumgarner's characteristic poise deserted him after he heard the news, as Cain described it.
"He was kind of in shock a little bit, I think," said Cain, who pitched six shutout innings at Los Angeles in last year's opener. "It's going to be a cool moment for him. I think he's going to really enjoy it."
Despite being only 24, Bumgarner is no stranger to big games. In 2010, he earned the decision in the Giants' Game 4 Division Series clincher at Atlanta, then became the fourth-youngest starter to record a World Series triumph by pitching eight shutout innings in Game 4 at Texas. Two years later, Bumgarner worked seven shutout innings in a Game 2 victory over Detroit in the World Series.
Cain, who finished 8-10 with a 4.00 ERA last season, admitted that Bumgarner outperformed him in 2013.
"I was not happy with the way I pitched last year," Cain said. "That's something I'm going to [address] this year and make sure it doesn't happen again."
Bochy did not finalize the rest of the rotation, noting that it still can be tweaked between now and the start of the season. Tim Hudson and Tim Lincecum are currently the third and fourth starters, respectively. After the season-opening, seven-game trip to Arizona and Los Angeles, Hudson thus would start the April 8 home opener at AT&T Park against the D-backs, barring changes.