The request caught Mark Montgomery off guard. He shifted forward in the chair by his locker, and shook his head. "Uh ..." he said. He was talking about the grip for his slider, the breaking ball that’s turned Montgomery, a 22-year-old righty, into the Yankees’ top relief prospect. As he scorched through the lower levels of the minors, opponents often pestered him about the secret behind his devastating offering. One day last week, a stranger asked to see the grip. Montgomery blanched. "I’ve kind of been told not to do that," he said. It would be technically inaccurate to call Montgomery’s slider a secret weapon. The pitch enticed the Yankees to draft him in the 11th round of the 2011 draft out of Longwood University in Farmville, Va. He dominated during his first two professional seasons, with a 1.65 ERA and 14.6 strikeouts per nine innings, and reached Double A last summer. One rival talent evaluator who watched him last fall called the slider "the best breaking ball I saw" in the hitter-friendly Arizona Fall League. Montgomery fanned 19 batters in 10⅓ innings. "And they knew it was coming," the executive said. Tuesday afternoon, the talk inside manager Joe Girardi’s office centered around the difficulty of succeeding closer Mariano Rivera. When Rivera retires, David Robertson looms as his replacement. Another former heir to that throne, star-crossed righty Joba Chamberlain, will be a free agent next winter.