If his primary position is going to be SF, he is going to be relied upon to help rebound and drive the ball. His ability to establish position for rebounds and ability to finish at the rim absolutely can be affected by his level of strength and mass.
I agree his skill and effort will dictate his success, but his ability/inability to add strength could be the difference in good and great. I'd feel less strong about if he were primarily a guard, but if we are moving towards a lineup that utilizes multiple 6-5 to 6-8 guys, he is going to have to do a lot of work at the rim. Fortunately, aggressiveness isn't an issue for him.