The decision was made in the aftermath of Bard's last start Sunday in Toronto, in which he walked six and hit two batters in 1 2/3 innings.
The Red Sox activated outfielder Darnell McDonald to take Bard's roster spot.
Bard will continue to work as a starter in Pawtucket, at least for the time being, Valentine said. Daisuke Matsuzaka is pitching Tuesday night in Pawtucket and will be limited to three innings; a decision on whether he will take Bard's spot in the rotation Saturday will be made after that, the manager said.
"Ben (Cherington) and I met yesterday and we met with the coaches today," Valentine said. "We felt it was the best thing for the organization and Daniel to work on his mechanics and not have to worry about his big league record and teammates. He thinks it will be a quick turnaround."
The pitcher was opposed to the demotion, Valentine said. "He thought he could find it on the fly. But we're taking a conservative route."
Bard, who turns 27 on June 25, never has been back to the minors since making his major league debut on May 13, 2009. His conversion from shutdown setup man to starter has been rocky. In 10 starts and one relief appearance, he has a 5-6 record with a 5.24 ERA, and has walked 37 batters in 55 innings.
After his outing in Toronto, Bard talked about the difficulties he has encountered in making the transition.
"There's no reason the way I threw the ball out of the bullpen shouldn't translate to starting," he said after Sunday's start. "I know it's not exactly the same. We've tried to change too many things. We just need to get back to being simple."
Bard's lack of command is a direct consequence of his inability to consistently repeat his delivery.
"He has three little checkpoints he is trying to correct at the same time," Valentine said. "He's going to try to simplify a little."
Valentine said there will be another meeting Wednesday to map out a schedule for Bard in Pawtucket. The player was not at Fenway Park when reporters entered the clubhouse Tuesday; his locker was cleaned out.
Valentine said there are no physical issues hindering Bard. "He feels real good," he said, adding that while Bard didn't feel a demotion to Pawtucket was warranted, he was in a "real good frame of mind."
Valentine was asked if Bard is overthinking matters.
"I think a player has to feel it," he said. "In Daniel's case, he wants to think it and feel it. The thinking part may get in the way.
"Pitchers like to make everything a little more complicated. It's a feel we're looking for. You don't have to write a thesis on this. Feel it."
Valentine acknowledged that in preparing Bard to become a starter in the offseason and spring, there may have been "a little bit of overload. A lot of things were being digested at the same time," he said. "It may have been too much."
Beyond Matsuzaka, the Sox do not have a great deal of starting depth, which is a factor in keeping Bard as a starter in hopes he can resolve his problems.
Information from ESPNBoston.com's Joe McDonald was used in this report.