Neville will become Rooney's coach for England friendlies against Republic of Ireland and Brazil, said the striker needed to sort himself out.
"Wayne is an emotional lad and you can see in the last couple of months he hasn't quite been himself," said Neville.
"Wayne Rooney wears his heart on his sleeve. He fights every refereeing decision. But he has been a bit vacant on the pitch. You can see something is wrong.
"But there is nowhere to go from here. I have seen players join Real Madrid and other great clubs in Europe. But they always want to come back.
"If you play for this club for 20 years it is not a bed of roses. You struggle mentally at times. He will be going through a difficult moment.
"But in a couple of weeks he will be coming away with England. Then he needs to have the summer off and hopefully get his head back to being a Manchester United player."