A player's worth is not defined by simply how good they are, other than in very broad terms (i.e. Pogba is worth more than Huddlestone), but by what the market dictates. If United are willing to spend £100m on this player, then that is their worth. And their worth is more than just how they play, but also their marketability, influence on the club as a whole (does their presence inspire others to join?), and how their performances improve the financials of the club (such as winning trophies). It's stupid to accuse a player of not being "worth" that much, because by the very definition of paying it then it becomes their worth. United will pay what the market demands of them for the players they think will improve that position, so the only true question is: does that player improve that position or not? And the answer is quite clearly, yes. Whether or not other players would improve it more is a moot point and something for your lurid fanfiction.
I apologise for contributing to this insurmountably dull discussion.