Girardi said he had been thinking about breaking up the left-handed-hitting Gardner and Ellsbury.
"It makes it harder to navigate through our lineup," Girardi said. "We feel like it's a better situation for all of us. This could very well be similar to our Opening Day lineup."
Sanchez hit 20 homers in 53 games last season after being recalled from Triple-A Scranton/Wilkes-Barre in early August. Bird has shown a power swing during spring training after missing all of last season with a shoulder injury.
Adam Warren, who was in the mix for the fifth starter spot, will work out of the bullpen.