Neville believes Van Gaal will have to splash the clash if the Red Devils are to mount a title challenge.
"As a previous coach, all the old players leaving, Sir Alex Ferguson leaving, I understand the job that needed to be done," Neville told football news outlet Breathe Sport.
"It was the biggest rebuilding job in a long time.
"This coming season is the season Van Gaal will be judged.
"Supporters will want more next year. Man United still needs to spend £100 million to have a chance next year."