With Morris' toughness and ability to guard out to the perimeter, I am not at all worried about his rebounding. It's great that AB had a higher rebound % than Morris, but does anybody actually think AB is likely to outmuscle/outposition him for a tough rebound?
Guards all around the league have been cleaning up the easy boards in recent years. Look at guys like Westbrook and Harden - I am going to go out on a limb and say they aren't two of the best rebounders in the league.
Morris won't be heavily depended on for scoring, but his presence as a big (yes, he will be a big in Brad's system) will be very versatile and I suspect, effective.