Manager Don Mattingly announced the move Saturday before the Marlins played the Milwaukee Brewers. The decision was not Alcantara's, according to Mattingly.
"Sandy was, obviously, he's one of those guys that wants to keep going," Mattingly said.
Alcantara (14-9) had a 2.28 ERA over 32 starts and leads the majors with 228⅔ innings. He pitched his sixth complete game of the season Friday night. He allowed one run and five hits through eight innings in a 1-0 loss to the Brewers.
Alcantara struck out 207 and walked 50 batters. He also had a complete game June 29 when he beat the St. Louis Cardinals 4-3.
"It's pretty incredible really what Sandy's been able to do," Mattingly said. "There's no reason to have him continue this year. We got to take care of Sandy first."
Alcantara would have started Wednesday in the Marlins' final game of the season against the Atlanta Braves.
"We got to do the best thing for Sandy, and we felt like this was it," Mattingly said.