Don Nelson is the winningest coach in NBA History. He turned teams into contenders everytime he took over as a coach from Milwaukee Bucks with Sidney Moncrief, who upset happen to sweep the C's back in 1983 and was a legit contender with that team, the Run TMC Warriors, Dirk-Nash Mavs and even pulled an incredible upset against the same team back in 2007 with Warriors.
Around the 1996-97 season, Nelson was fired by Knicks after his failure stint with the coaching job. Meanwhile, ML Carr tanked the season hard, finishing with season with the worst record in franchise history. He would resign after this season and Pitino took over.
Nelson then sign with the Mavs which then developed the team and its franchise player Dirk into an all-time great. After 2001 season, the team never looked back and won at least 50+ win every season even after his departure.
I think signing him would have helped the C's back then. Not only he have a great resume, but he also have a history with the team as a player and have his jersey retired in the Garden rafters. I think Billups and Walker would have developed better under Nelson than under Pitino.