Gary Neville spoke at the opening of Salford's new stadium about how Manchester United are title contenders for the first time in four years. (1:12)

Luke Shaw has found himself on the sidelines at Manchester United because he has not proved to Jose Mourinho that he can be trusted, according to Gary Neville.

Shaw is fit again after recovering from an ankle injury over the summer but has made just one substitute appearance so far this season.

The 22-year-old travelled to Lisbon for the Champions League clash with Benfica this week but was not named in the squad.

Neville branded Antonio Valencia a "wonderful" right-back after United's 1-0 win in Portugal, which kept Mourinho's team at the top of Group A, but he admits Shaw has work to do to nail down a spot at left-back like Valencia has done on the right.

"Luke has got to deliver the same level of performance as Antonio Valencia," said Neville, who coached Shaw during the 2014 World Cup as Roy Hodgson's England assistant.

"He's a long way away from that at the moment. You have to build trust, like Valencia has done these last two seasons.

"I trust Valencia when I watch him and you know he's going to have a good game, you don't even question it.

"You don't think that he used to be a winger or that he might not be good defensively. He can do it and he's shown he can do it.

"That's what Luke has to do but he hasn't proven that yet, even though there is a huge talent there."

Luke Shaw is a long way from becoming a regular for Manchester United, Gary Neville has said. Martin Rickett/PA Wire

The end of Neville's United career overlapped with the start of Valencia's following the Ecuadorian's move from Wigan in 2009.

Sir Alex Ferguson signed Valencia as a winger and Neville admits his transformation into one of the Premier League's best right-backs has taken him by surprise.

He said: "He was always strong and he could defend well, even when he was a right winger, and he was a dream to play behind. But did I think he was a right-back? No.

"Even when he first went to right-back I thought it would be a temporary move, but now United don't need to sign a right-back.

"Antonio Valencia has been outstanding and has been now for two seasons.

"He is one of the best right-backs that I've seen in the last couple of years, definitely. For me, along with David De Gea, he's been United's best performer for two seasons.

"He's fantastic and rarely makes a mistake. He can't be considered a makeshift right-back anymore he's now a right-back, a wonderful right-back."