He has been criticised of late after insisting his side needed to sign even more players to compete with runaway league leaders Manchester City.
Mourinho has spent almost £300m since arriving at Old Trafford.
But in a series of tweets, Neville said the Portuguese needed to be backed with more money.
He wrote: "On United. We should be closer to City irrespective of their incredible run but city did have a better squad than united when Jose came in.
"The recruitment under LVG / DM all over the place. The club should be signing players that suit the values not being taken from one extreme to another by managers with differing principles.
"It's a very costly exercise and means the waste is huge.
"The players signed by Jose are much better and more aligned to the clubs values. He should be supported in the TM with more players that meet the club ethic."