Manchester United’s Luke Shaw (right) looks to avoid a tackle from Andrea Ranocchia, of Inter.

Luke Shaw insists he has no problem putting the hard yards in to make sure he is fully fit for his first season as a Manchester United player.

In a frank interview following United’s penalty shoot-out win over Inter late Tuesday, Shaw conceded manager Louis van Gaal had every right to criticise him over his lack of fitness.

And the 19-year-old admitted he underestimated just how challenging it would be to make the step up from Southampton to Manchester United.

Van Gaal revealed at a press conference on Monday that he has had to devise a special training programme for Shaw, who turned up for pre-season out of shape after three weeks off.

The Dutchman was clearly not too happy with Shaw given that he had only just sanctioned the teenager’s expensive transfer from the south coast.

Shaw has no qualms with his manager’s public criticism of him.

“Obviously, I have seen that, and it’s something I totally agree with,” Shaw said when asked about Van Gaal’s comments.

“To play in this formation you have to be really fit. I am fit but I am not at the high level he demands.

“I have got to get fitter to get up and down (the wing). It’s something I have got to work on.”

Shaw became the fourth most expensive defender in history when he swapped Southampton for United in a deal that could cost the Old Trafford club £28 million.

Although he played for England at the World Cup, it is worth remembering Shaw only made his full Premier League debut in November 2012.

Shaw admits he miscalculated just how physically demanding it would be to step up and become United’s first-choice left-back as part of Van Gaal’s new 3-5-2 system.

“It was a surprise. I didn’t come into United thinking it would be much harder,” Shaw said.

“That’s something that I think I made a mistake on.

“I thought it was just going to be the same but it’s the biggest club in the world and there is world-class players in training.

“That is something that is going to push me on and it’s something I am looking forward to doing.”

To show just how much work he thinks Shaw needs to do, Van Gaal made the left-back partake in a gruelling fitness session on the morning of the win against Inter, which came on penalties after a 0-0 draw.

The England defender only made it onto the pitch when youth product Reece James pulled out of the match with an injury.

Shaw and the rest of his United team-mates will fly to Detroit today ahead of their weekend clash against Real Madrid, which is expected to take place in front of a crowd of over 100,000 people.

A win for United, who beat Roma in their first match, will see them qualify for the final of the International Champions Cup, which takes place on August 4 in Miami.

Meanwhile, Inter’s new captain Andrea Ranocchia told reporters the Nerazzurri are becoming a strong team.

Inter have beaten Real Madrid on penalties and lost to United the same way in the US but despite varied results, defender Ranocchia insists the San Siro outfit are developing into a formidable team.

“We are becoming a strong team,” he told Inter Channel.

“We showed against United that we are a real team and that we can compete on a level playing field.

“I didn’t think they created much and a lot of that is down to the defenders. We looked good in possession, but have another important test against Roma soon.”