I highly doubt it. But let's phrase it like this (also as a comment on
@Castia above stating merely the same): it shouldn't be. Succes nowadays isn't determined/achieved by just winning. Winning can be incidental. What's more important is creating a foundation to fall back on. It creates continuity in the process of achieving succes as well as a safety net when succes isn't achieved. Concepts like a clear philosophy implemented through your whole club (etc.) are examples of that.
To be honest, that's the main difference between clubs like Liverpool/City and Man Utd at the moment. Ruud doesn't fit in that picture. He is not tactically developped in a modern way with lacking clear philosophies and ideas as an head coach to identify with and build on as an organization.
He isn't ready for the job and a lot of people have serious doubts he will ever be. Tactics are nowadays more important and complex than ever and that part lacks Ruud the most. However, he has strong sides too. He is motivator, a gentlemen, a great endorsement and his experiences as an ex-prof have benefits in developing players. For example, Xavi Simons was a player who was satisfied with Ruud's capabilities as a mentor for young players. Living for the sport, how to get better, to find and exploite the small margins of success on the highest level. I think someone like Rashford could benefit from the presence of Ruud. Most people, just like me, think he is better in a role at the front office, a specialist trainer or in this case as an assistant trainer instead of being the head coach.
But yeah, we are all a bit surprised that United considers him as an assistant coach. Although I have to admit, everyone loves Ruud (you just can't hate the man). It is a huge but also a very cool step to make. And learning form Erik ten Hag, who i consider as a good coach based on his Ajax period, doesn't seems to be a bad thing for his development.