The Sporting Lisbon star is coming to the Red Devils, with the two clubs having agreed on a transfer.
Bruno will have his medical in the next days and sign his contract.
He may even be available for their Premier League game against Wolves at the weekend.
But Neville thinks it will be a while before Fernandes can show his best form in England.
Fernandes managed 13 goals in 22 games for Sporting in the Portuguese league this season.
Speaking to Sky Sports, Neville said: "I've not got huge expectations of Bruno Fernandes in terms of this next few months.
"I think it'll take him a little time in England to settle in. He'll need some time.
"But they've got a good, quality player into the building. Ultimately we shouldn't be sat here thinking he's going to be amazing straight away.
"Obviously it's very difficult to hit the ground running in the January transfer [window]."