Manchester United legend Gary Neville believes losing David de Gea will be a huge blow.

With Real Madrid again eager to take the Spain No.1 back to his homeland, there is a worry that United may allow De Gea to leave, two years after his last dalliance with the 11-time European champions.

"De Gea was fantastic [against City] in the bits of work he had to do," said Neville, speaking on Sky Sports. "He was calm, composed, authoritative.

"If United were to lose De Gea, it would be a big problem. It would not be easy to replace him. He's right at the top level now.

"You're talking about [Thibaut] Courtois, [Manuel] Neuer, that level of goalkeeper, that's where he is now. I think he is better than [Hugo] Lloris.

"United have got to sort that issue out. They've gone through the early years of flapping and inexperience [with him]."