Giants manager Bruce Bochy stressed the importance of Sanchez's getting regular work to "get back to where he was when he came up last year."
Sanchez had 21 strikeouts in 14 innings, but also an ERA of 5.52. He gave up a walk-off home run to Carlos Lee in the 10th inning loss at Houston on May 15.
Sanchez will continue to throw out of the bullpen in Fresno, Bochy said.
"But we want to do it on a consistent basis, give him a couple of innings every two, three days," Bochy said.
Placing Sanchez on a semi-regular schedule, which would allow him to develop properly, is near impossible at the major league level, Bochy said.
"You're on call, so it's hard to work in the bullpen," Bochy said. "Down there, if you want to, you can give him a few days off to work on something specific."
The manager said that what Sanchez must work on is his control.
"I think he's lost command of his offspeed pitches," said Bochy, who added that Sanchez must also better locate his fastball so that he doesn't have to try to overpower hitters.
Bochy said he expected Sanchez to "get back up here and probably won't see the minor leagues again," and that he still envisions him being a starter in the big leagues one day.
The demotion of Sanchez leaves the Giants with two left-handed relievers, Steve Kline and Jack Taschner. Heading into Monday, Kline had pitched nine times in the Giants' last 20 games and given up six earned runs in five combined innings.
ONE-HOPPERS: Dave Roberts swung the bat lightly for the first time since undergoing surgery on his left elbow on May 11. Roberts is expected to miss another three-to-five weeks. ... Rich Aurilia started at third base in place of Pedro Feliz, who was given the day off. ... Bochy said he would rest shortstop Omar Vizquel either today or on Wednesday.