Brad Stevens is learning to deal with what every NBA coach must as he takes the step from the college game. But beyond the longer schedule and longer games and a schedule designed by the Marquis de Sade Travel Agency, the Celtics leader has yet another challenge. While most coaches don’t need to worry much about the end of their bench because it’s inhabited by raw youngsters who aren’t ready for prime time (think Fab Melo and Kris Joseph last year), the Celts have veterans who’ve been largely on the outside of the rotation looking in. Twenty-two-year-old Kelly Olynyk and 21-year-old Jared Sullinger are regulars, and Stevens has been looking to get Phil Pressey, 22, more time. Twenty-five-year-old rookie center Vitor Faverani is in the mix, and Avery Bradley, 23, is a starter. That means people like Kris Humphries and Keith Bogans and young (25) but ready MarShon Brooks have mainly been spectators. The situation came about because the Celtics had to take back good-sized contracts from Brooklyn to balance Paul Pierce and Kevin Garnett’s trip in the opposite direction, and it’s left the locals with some guys, including Gerald Wallace, 31, who are almost certainly not part of the long-term solution. “First of all, there just hasn’t been separation, so we haven’t been able to say clearly on a given day that Player A is better than Player B,” said Stevens before taking his team south for a back-to-back in Orlando tonight and Miami tomorrow. “But the old guys have handled it great. “The most respect I can have for an athlete is somebody who sits and comes in — and doesn’t know they’re coming in, and does their job well. Hump’s play (Wednesday) night was a great example of that. Keith Bogans every day is a great example of that.