ralf rangnick is an extremely competent manager, you can but admire his knowledge and expertise.
that said, i don't think this is a particularly good fit. he shines when he can implement his vision, is in full control of the club and its development. i don't think managing big egos (both in players as well as club bosses) is his strong suit, and that seems to be one of the core issues at united, apart from the tactical shortcomings that solskjaer's team displayed.