I think with young players coming into the team you have to measure their performance on two levels. What they bring to the team now, and what they can possibly bring in the future. I think he differs from those players you mentioned because they probably looked better in the few performances in LVG's system with his coaching compared to what they ever showed previously in the U21's. Van Gaal was very good at utilising players for a specific purpose and it gave a good platform for these kids. The excitement we got from that was cool, but we all knew these players had a long way to go, as has Williams.
But based on the qualities he has shown to have in the U21's alone, I rate him higher than those players. I think he has taken to the PL-level incredibly fast, way above expectations but it is clear to me he possesses a lot of qualities he just isn't able to perform at that level yet. There is no shame in that what so ever, as the qualities that make him able to play at this level at 19 is something we do not usually see. He is technically good, composed on the ball and good passes. He also manage to play his game and keep up with the physicality and pace at that level which is very good. To me, he is showing maturity, discipline, heart and professionalism. These are qualities that might not always make him stand out right now, but things that makes him seem like a certainty for the future.
While there is no guarantee for anything, I'd like to value these traits as I don't see them in young players that often. He has obviously been a good student, and his understanding of his game and his role already is vital, imo. If that is the things that comes easy to him, it means he can keep a certain level of consistency from a young age and focus on developing other aspects of his game while playing at a high level. It is a great platform for reaching the top, and while there is no certainty of him becoming the next Lahm the signs are more than encouraging. He still has a lot of qualities to add to his game and things he haven't shown at this level yet.
I want to add that going up against Mahrez is very hard. I didn't like how he approached him defensively 1vs1 but tbf he was isolated with him with a lot of space behind him, so I do understand how he couldn't be more aggressive against City. He obviously lack experience so certain situations at the top level will be completely new to him. Everyone can afford to have some patience with kids on these things especially. If he'd look totally out of his depth or affected by mistakes I'd be the first one to mention it because it goes against the things I've summarised above.
I also think he wasn't as good offensively against City because we lacked an attacker and him and Shaw took turns keeping width. It got congested on that side as it always gets when Shaw plays, and especially with both of them there. I didn't enjoy that aspect at all. Williams has usually been a breath of fresh air, being a lot quicker and dynamic in his movement to create more assuredness through that side, which has helped it not get congested and crowded. Wasn't the case against City though. I also want to add that I think Shaw has been decent the last couple of games as well.