Ajax, and the Dutch in general, have a sense of arrogance and belief based on past achievements and ability - they will not give a fuck about United or Mourinho, they will expect to win. And if United want to compare CV's, they can do that as well
I don't know who will win but I can imagine it will be another game in which United are terrified to lose and Ajax will camp in the United half
Let's look at who United have drawn/lost against in this tournament:
Feyenoord
Fenabache
Rostov
Andrerlect
Celta Vigo
I'm not sure how they got on against St Ettien, but you get the picture