Mourinho is used to have a short number of players available, because as he stated today his teams have not that many injuries, and so that there's competition among first team players and substitutes and keep these interested.
In here, we might see that number raised a bit because he's getting to know them and also United's stated youth policy. But I can't imagine Mourinho having 3 players per position all season. In his last winning season at Chelsea he had just 24 players (only 3 goalkeepers and strikers, the rest with 2 each).
So, I'm surprised a lot of people here are expecting him to keep most to all of Van Gaal's players, including the youngsters that entered the first team last season, more the 4 priority additions, which amount more or less to 33 players!
Even if Mourinho maintains an extra man for each line, I can't imagine him having more than 26 players training come 31st August. Considering this, I think at least 7 might have to be sold or loaned, and I expect them to be the following:
Rojo
McNair
Varela
Fellaini
Young/Depay
Mata
Pereira/Januzaj
Wilson
The funny thing is that Mourinho might use this in his favor. As he stated in the press conference, he's counting on adding to the squad if some players want to leave. So, for instance, if Mata is sold, he will demand for a new #10 of his preference instead of keeping Pereira. (And, by the way, here's when Mendes players might start to appear, if Mourinho isn't breaking with him.) The same with high profile players like Rojo, Blind, Darmian, Fellaini, etc. But come the end of August, he sure will going to get rid of the excess baggage.