Brendan Galloway was causing Mata problems when he dropped deep to help in defence and Everton posed a real threat down the left hand side because of it.
In the second half after Jesse Lingard replaced Mata, that threat diminished because the youngster had the pace to match Galloway and it really helped United stay stronger at the back.
LvG: "Juan Mata's substitution was tactical. He has no injury." #MUFC
— Shamoon Hafez (@ShamoonHafez) October 17, 2015
As good as United were in attack against Everton, the defence deserves plenty of praise. Chris Smalling was imperious at the heart of defence, Phil Jones also played very well and although Marcos Rojo was shaky at points, David de Gea was there to clean everything up whenever Everton got a chance.
Mata has been excellent all season but the decision to take him off proved to be a shrewd move by van Gaal. The United boss has had plenty of criticism for some strange substitutions at points this year but taking Mata off was a fantastic tactical move.