This is not nostalgia. This is necessity. This is not thanks for the memories. This is about making more memories. This is not Old-Timer’s Day on endless loop. This is asking for at least one more year of vitality. There was a vintage quality to having Derek Jeter, Andy Pettitte and Mariano Rivera all on the field in the same game last night, considering the 15 combined rings and 121 years on the planet. But one look at a thinner Yankees roster than usual — anorexic with Curtis Granderson, Mark Teixeira and Alex Rodriguez sidelined — means the lingering pieces of the Core Four “really matter” in Joe Girardi’s words. The trio actually appears as central to the Yankees’ success as they were many years and hairs ago in 1996. The retired member of the quartet — Jorge Posada — was back in camp as a special coach, chatting up his pal, Jeter, behind the batting cage and helping with pre-game preparation. Jeter, though, is again going to be the leadoff-hitting shortstop. Pettitte the No. 3 starter. Rivera the closer.