United drew 0-0 with West Brom last Saturday – their 11th draw of this season.
On Monday, Mourinho admitted that the Europa League represents their best chance of qualifying for the Champions League.
After the West Brom game, the former Chelsea boss blasted his attackers for not being consistent and Neville is expects another big summer.
“In some ways, when you can easily identify your weaknesses and where you need to strengthen, that’s a good thing,” Neville told Sky Sports .
“At this moment in time too many mistakes have been made and I think Jose will know that. His frustration with his strikers after the game on Saturday was real.
“He’s lining it up, you can see it already. There’s talk of Griezmann coming but he’s lining up for another big summer. It’s not just the attack where he’s short.
“He needs a centre-back and a holding midfield player. He needs five or six to come in and probably two or three will go.”