Brown is a lot more athletic than the Stanimal, but unfortunately, Johnson is a better prospect in every other way. He was an excellent NCAA defender, while Brown showed flashes but was inconsistent. Johnson got twice as many steals as Brown; steals are incredibly predictive of NBA success. More importantly, Johnson was a more efficient scorer with far better advanced stats. An OBPM of 4.8 vs 0.3 for Brown. 55% vs 52% TS%. 22.5 vs 17.6 PER. 37% vs 29% 3p%.
There is an argument to be made that Brown would have been better on a team with better spacing, but he himself was part of the reason for poor spacing, and the Celtics don't exactly provide a ton of floor spacing either.