A lot will depend on finances and how much he's backed. Utd aren't exactly in the top tier anymore of being able to afford or persuade the elite players to come to Utd.
That in itself is a major hurdle to compete for the PL title, or even get top 4. When you also think much of this current decade is going to be blighted by recessions, financial concerns etc, this will impact utd more as they rely on revenue to compete with the oil teams who have no such concerns.
So even if ETH is a top coach, he's only as good as the players at his disposal. People saying what he achieved at Ajax etc, but the Dutch league isn't comparable with the PL. Heck, Steve McClaren won a league title there and yet never really achieved much as a manager in England.
I don't think he's going to succeed in winning or even mounting a title challenge. The demands here are too great and the club is just not run well enough to create the environment for success. Mourinho was the best we've had and even he knew he'd done as much as is possible. I don't think ETH is going to come close and this will end in tears eventually.