LONDON, July 27 (Xinhua) -- Manchester United's leftback Luke Shaw has endured a difficult four years since signed for the club as an 18-year-old in 2014, but he has spent the summer working to put things right.

At the time of his move from Southampton, Shaw was the most expensive leftback in the world and it was thought that he would occupy the role for the Red Devils and his country for the next decade.

However, that hasn't been the case due to injury and fitness issues and Shaw struggled to win the confidence of former boss Louis Van Gaal, who described him as "fat", while Jose Mourinho did not use him on a regular basis either.

Nevertheless, Shaw has started all of United's friendly matches in pre-season, something he puts down to carrying out a demanding fitness regime in pre-season.

"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," Shaw said in an interview in the Guardian newspaper.

He denied he had ever been fat, insisting he was "bigger built"; but he has been working hard on his fitness over the summer, even going to Dubai to get into shape.

"We had to wake up early because it was hitting 45 degrees by 10 o'clock - it was quite painful the first day as we did do it in that heat. I would train and go in the gym. Then relax during the day and run on the beach and do core work in the evening before the sun went down," he said.

The 23-year-old said Mourinho felt very positive about his efforts and with Ashley Young expected to miss the opening match of the new season after playing in the World Cup, he now has the chance to kick off the season on the pitch rather than in the stands.