Jeremy Hefner earned his first win of the season, and could be solidifying his rotation spot. With Zack Wheeler on the verge of joining the big club, a Mets starter will have to be dropped from the rotation, and — barring injury — Hefner and Dillon Gee would be candidates for the bullpen. Last night, Hefner delivered a solid start in the Mets’ 9-4 Subway romp, surrendering three runs in six innings. While Ike Davis and Ruben Tejada are in danger of minor league demotions, Gee appears to be a lock to remain in the majors, even if he loses his rotation slot. Gee’s 6.34 ERA is the second-worst among NL qualifiers, but manager Terry Collins told The Post yesterday Gee is not in danger of being shipped to the minors, even if Wheeler joins the rotation and Hefner stays in it. Rather, Gee could be a candidate for the bullpen. This is partly because Gee, who will start tonight’s Subway Series finale, has pitched in relief before. In 2011, the right-hander worked three games out of the pen, allowing four runs in 4 1/3 innings. Gee said moving to the pen has not been discussed with him. Nevertheless, one of Gee’s problems this season has been collapsing in the middle innings after starting well. So working in relief may better fit. “We’ll probably have to re-evaluate if something doesn’t change here soon,” Gee said. To stay in the rotation, Gee might need Hefner to start struggling. Making his first Subway start last night, Hefner was handed a 5-0 first-inning lead and never had it threatened. In his last seven starts, Hefner has a solid 3.98 ERA with just 49 combined hits and walks in 43 innings. He credited the stretch mainly to “pitching inside effectively.”