Just another day in tropical paradise for the Mets. The already beleaguered team can only hope to have enough healthy arms to form a rotation, after Shaun Marcum and Jeremy Hefner yesterday joined the ranks of those seeking medical attention. Of the two, Marcum’s availability to start the season seems more dubious. The veteran right-hander has “neck discomfort,” according to the team, and began treatment with an anti-inflammatory, a day after his bullpen session was curtailed because of soreness. Hefner was hit in the pitching elbow by Carlos Beltran’s one-hop comebacker in the fourth inning of the Mets’ 11-4 exhibition loss to the Cardinals and was diagnosed with a contusion — X-rays were negative — but said he expects to be ready for the season. But an already thin rotation minus Johan Santana is murky behind Jon Niese, Matt Harvey and Dillon Gee. Manager Terry Collins said he’s still counting on Hefner to pitch the fourth game of the season, but isn’t so certain about Marcum, who has been scratched from his scheduled start tomorrow. “We’ve got to figure out not only, if Shaun can’t go, who the fifth starter is and where do we set him up,” Collins said. By default, the choice would almost certainly be lefty Aaron Laffey, the manager acknowledged. Laffey was reassigned to minor league camp on Sunday, hours after making his first start of the exhibition season — in which he flopped against the Cardinals. Though the Nationals yesterday released Chris Young — a favorite of Mets general manager Sandy Alderson — the veteran righty likely won’t be pursued by the Mets, according to a club source. Also not an option is top pitching prospect Zack Wheeler, who the Mets believe still needs seasoning at Triple-A Las Vegas. “There’s a reason we sent [Wheeler] out,” Collins said. “He needs to go to Triple-A, he needs to face hitters in Triple-A.”