Tatum's okay, but his foot speed is so much slower than Jackson's.
You don't refuse to draft someone just because the won't work out for you. There is a ton of game tape to evaluate him.
If report that Celtics turned down Fultz in part because he lacked explosiveness (which I find weird), then obviously they favor a Jackson over a Tatum.
Of course if we give the pick for Butler (ugh, I hope not), all this is moot.