"We're going to follow that to the tee and make sure he is built up and ready," manager Dan Jennings said. "We're looking sometime around July 1."
Initially, Miami had been using a window of June 15-July 15, or around the All-Star break.
Fernandez, who underwent Tommy John surgery on May 16, 2014, is slated to begin his rehab assignment on Saturday with Class A Advanced Jupiter at Port Charlotte, Fla.
The hard-throwing right-hander looked in midseason form in his extended spring outing, blazing a fastball clocked as high at 97 mph. How strong he looks raised the question of whether he is ready sooner rather than later.
If July 1 is his return date, it would be against the Giants at Marlins Park. The new window is more in the July 1-7 range.
"That's the plan," Jennings said.
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.