AWB is a much better right back than either Dalot or Williams, and I really don't see either getting up to his current level.
Ideally, I'd have preferred selling one of those out, and loaning out the other this season, and brining in someone like Trippier as an experienced, more attack minded fullback to challenge AWB for the spot and to be a genuine starting option that I'd have faith in playing well in a run of starts.
I really don't see that in Dalot or Williams - which is a worry if AWB suffers injury or loss of form. I see more 'potential' in Dalot than Williams as the understudy. However, he's 22 now, and still doesn't have any attribute to a high enough level that you'd need for a Man Utd first choice RB.
He's not reliable defensively. And while he gets forward reasonably well, his attacking end product is limited. He's OK on the ball and linking up in attacks. And can put in a decent cross at times. But he doesn't really hurt teams going forward very often, and isn't reliable defensively, so I'd have no faith in him delivering strong performances should he be needed for a series of starts this season.