Rubin says Dillon Gee would pitch their series opener against the Marlins so to split him from Marcum. But they could use Niese on the fifth day instead of Jeremy Hefner because of the off-day which follows the season opener.
The Mets technically don't need a fifth starter until April 10 against the Phillies. They have an off-day on April 11 as well, and so if Johan Santana is ready, they could slot him into the rotation on April 16 in Denver. But given the altitude and the potential for winter weather there in mid-April, I could see the Mets pushing Santana back to April 21 against the Nationals at Citi Field. That way, he can spend more time preparing in Port St. Lucie, and he might have better conditions to pitch under later in the month at home.