Sanchez has allowed 43 hits and 19 walks in 30 innings this season. He's 2-1 with a 4.80 ERA in six starts.
"We felt as an organization that he needed to go down and get some things straightened out," manager Fredi Gonzalez said. "It's tough to make adjustments at the big-league level and work on things.
"There's something not right. But I feel really confident he can fix it," Gonzalez said.
The Marlins reinstated right-hander Jorge Julio from the disabled list. Julio, who has a 19.06 ERA in seven games, lost the closer's job before he went on the disabled list on April 18 with a strained right calf.
Sanchez's no-hitter Sept. 6 came against Arizona in his 13th major-league start. It was the first no-hitter in the majors in 2½ years.
He finished the season 10-3 with a 2.83 ERA.
"Last year everything was going great for him, and there wasn't a lot of adversity," pitching coach Rick Kranitz said. "Now there is. This is the lowest point of his major-league career, but we have to see through it."
Sanchez has struggled with his command this season and failed to challenge hitters the way he did last year. Still, the demotion came as a surprise.
"He took it hard, and he should," Kranitz said. "He believes he's a major-league pitcher. I believe he's a major-league pitcher. But we have to do what's best for his career, and right now this is it."
Gonzalez said he wants to give Julio another shot at the closer's role -- but not right away. Rookie Henry Owens began the weekend 3-for-3 in save chances and will keep the job for now.
Julio, the Marlins' third highest-paid player at $3.6 million, said he received advice from a sports psychologist and others while on the disabled list.
"I talked to a lot of people," he said. "It helped me a lot. Everybody tells me, 'Take it easy, stay focused and keep going.' I feel right now like a new Julio."