Jon Niese admitted the obvious yesterday. “I don’t think we really have an ace on this staff,” Niese said. “Everybody contributes equally to this rotation and that’s all we’re really counting on.” With Johan Santana out for the year because of upcoming shoulder surgery and R.A. Dickey with the Blue Jays, the Mets are left without a clear No. 1 starter. So while Niese will take Santana’s spot on Opening Day today, the Mets are still waiting for someone to take Santana’s place at the top of the rotation. “We just have to pitch quality innings and keep the team in the game,” Niese said before the Mets held a voluntary workout at Citi Field. “It’s a bummer to hear about [Santana], but we have to move forward with the guys we have.” Guys like Niese, and especially Matt Harvey. “Sometimes you look at that guy every fifth day and you know what he’s going to do,’’ manager Terry Collins said. “Well, you know what? It’s Jon Niese’s turn. He’s now that guy. It’s going to be Matt Harvey; it’s going to be Dillon Gee and Jeremy Hefner. It’s time for those guys to be the mainstays.” Easier said than done. Niese pitched well a year ago but hasn’t won more than 13 games in a major league season. And Harvey is about to start his first full season in the majors. “They need to have the mindset of a No. 1 and No. 2,” David Wright said. “I think Niese certainly has done that and Matt Harvey has done that. Those are two guys that have that confidence ... They truly believe that stuff-wise, they deserve to be top of the rotation-type guys.” And that’s even more important without Santana.