The Boss was calling. He needed advice from Donnie Baseball about his young shortstop, Derek Jeter. A triangle of Yankee greatness: George Steinbrenner, Don Mattingly and Jeter. “George called me that first year that I was going to retire,’’ Mattingly told The Post Wednesday about the phone call leading into the 1996 season. “He said, ‘Do you think Jeter can play short? Do you think he’s ready?’ “Here’s what I told him,’’ Mattingly said, letting the world in on this vital piece of Yankees history after the young Jeter had played 15 games in 1995, Mattingly’s final season. “‘If you take him, you have to leave him at short every day and let him go. You have to let him learn. Don’t yo-yo him up and down.’ ’’ Mattingly sharpened his words even more to the Yankees’ owner: “If you take him, live with him there, if you think he is going to be great.’’ With that, a Hall of Fame career was born. Told he made the right call, the Dodgers manager smiled and said: “Yeah, I feel good about that one.’’ On the day Jeter held his formal press conference at George M. Steinbrenner Field to announce 2014 would be his final season, memories came flooding back to Mattingly. Mattingly knew Jeter had a chance to be a special player in 1995 after he mentioned to Jeter how to improve his throwing.