While he dose need work on that end(not many rookies don't), Stevens said he was really impressed how much better he had gotten at D during the summer league.
Which means, he knows it's a weakness, and he's working on it, and he's getting results. Brown had an edge last season were he could play really solid to great D, but his offense was lacking in the beginning.
Tatum might have solid to good offense, but lacking on the defensive end. But coming off the bench, he should get playing time unless he really stinks it up. They'll need some scoring off the bench and as long as he can play solid D and stay with in schemes, he should be fine.