After last season's ECF it was obvious that the Celtics would have to move on from some players who had been great servants but would struggle to get beyond LBJ. When the trade that brought Morris to the C's was made I loved the fact he was a known effective defender of LeBron.
DA probably saw more in Morris than thinking this day would come and the team would need Morris to help blunt LeBron, though he must have also factored this into his thinking. The team needed to be longer and stronger when it came to playing the Cavs. Morris may have other shortcomings and certainly wasn't a perfect fit for the Stevens system, but he can be significant in this series.
At least partially I feel like this is the reason Morris is on the Celtics. He should be a factor.