I think Brown did pretty dang good last year when he played SG with the starters. Stevens was able to hide his rookie mistakes and deficiencies by playing him with starters. By the time the playoffs started Brown was guarding SGs and SFs well enough to be a regular contributor. Brown is your starting SG this year.
As for his handle, remember when Bradley took over for Ray Allen? It was because he proved he could play NBA defense. Bradley's handle and shooting back then sucked. But he improved into a great two way player. I see Brown doing the same. And with Stevens all switch system of defense, Brown being able to guard multiple positions, much like almost everyone else except IT, will be key to him staying a starter.
I am actually excited to see Brown become a 20 year old starter on this team. I expect big things from him and Tatum this year. I think they are that good. They will be inconsistent but good.