We've got a set budget and need at least 1 or 2 CMs and a RW. So yeah, it is either/or.
For January, perhaps.
However he is still a kid and has plenty of time to grow and develop, not to mention we will sign plenty of players during the course of his career at United (assuming he signs in January or in the summer).
When an opportunity to sign a really talented player comes along you take it.
We will fill in the gaps in midfield in the following transfer windows.
In my opinion you are taking a bit of a short term view whereas I am looking at the bigger picture and the next 5-10 years