Celts could have traded McHale to Seattle in early 89 in a package that would have wound up getting the #2 pick in the 1990 draft (GP.) They also could have traded him to Dallas in that same time frame for Detlef Schermpf, Sam Perkins, and a pick.
Had they done either of those trades, and then picked Tim Hardaway (who wanted to be a Celtic, was playing a position of dire need, was the slam dunk pick by everyone who watched, followed, or covered that draft) - they would have been competitive throughout the 90s with or without Len Bias and Reggie Lewis.