The move was first reported by ESPN The Magazine's Buster Olney.
Zito, who only 16 months ago signed a $126 million, seven-year contract with the Giants to lead their rotation, was informed of the move in a meeting with manager Bruce Bochy.
"I'm certainly not happy with it, by any means," Zito said. "But this is the bed that I've made. I have to lay in it for the time being and I have to overcome. I trust management and I trust what their decisions are."
The left-hander has lost his first six starts this season and has a 7.53 ERA that jumped considerably after Zito was tagged for eight earned runs in a 10-1 loss Sunday to Cincinnati.
The 2002 AL Cy Young Award winner with Oakland, Zito lasted just three innings against the Reds and was booed frequently by the crowd at AT&T Park during his shortest outing of the year.
"It's good sometimes just to back off," Bochy said. "It's happened to a lot of great players, position players and even pitchers. We just felt at this point it's time for him to sit back, miss a start and help us in the pen."
A three-time All-Star and San Francisco's Opening Day starter this season, Zito is only the third pitcher since 1956 to go 0-6 before May, joining Texas' Dave Stewart (1984) and Detroit's Mike Maroth (2003).
The durable Zito has made 247 consecutive starts without missing a turn due to injury, the longest streak in the majors. He made his only big league relief appearance last season.
Zito's next turn in the Giants' rotation would have been Saturday in Philadelphia. Instead, San Francisco plans on using Pat Misch against the Phillies. Misch was recalled from Triple-A Fresno when right-handed starter Kevin Correia was placed on the 15-day disabled list.
Because Zito started Sunday, Bochy indicated he wouldn't be available to pitch in relief until the Giants play the Phillies beginning Friday.
"We'll watch and see how things go," Bochy said. "Right now we're just going to leave it at, that he's going to be helping us out of the bullpen."
Buster Olney is a senior writer at ESPN The Magazine. Information from The Associated Press contributed to this report.