Manchester United defender Luke Shaw has hit back at his critics by insisting that he has "never been out of shape", while also vowing to prove himself to boss Jose Mourinho.

The 23-year-old has failed to hold down a place in the Red Devils' starting lineup since arriving in 2014 and is now into the final 12 months of his existing Old Trafford contract.

Shaw has regularly been questioned over his fitness levels, but the England international insists that the "negative criticism" aimed his way is misdirected, much in the same way that former teammate Wayne Rooney also had to fend off such accusations.

"I guess you've just got to take it because there is always going to be negative criticism and positive but both of them can make you stronger," he told The Guardian. "I've been unlucky because I've had a few ups and downs with different managers but I would say I've never been out of shape. Honestly, I feel really good and I'm raring to go, and the minutes [on tour] are only going to help me become fitter.

"People can say I'm fat but I know my own body. I always look big because I'm bigger built – I've got that Wayne Rooney type of body. I worked hard and not just for [critics]. I'm working harder than ever and in the first game I want to look 10 times better than in that picture.

"It would be easy for me to quit and say: 'I want to go.' Of course, if the manager comes and says: 'You're not a player for Manchester United, you're not a player for me,' then I'll accept that and find another place. I want to earn a contract. I don't want a contract because in the next year I'm a free agent, so they might look to tie me down.

"I know the club believe me – I've spoken with them, the manager, I've had discussions, meetings. If they really wanted to they could've cashed in. I'm going to fight for it this year and I want to be in that starting team."

Shaw has started all three of United's games on their pre-season tour of the United States so far.