One o’clock is nearing when Luke Shaw stands up and says: “Can I go to lunch because I’ve literally got one minute?” The respect the defender holds for Louis van Gaal and the Dutchman’s demand for punctuality is evident; to be even slightly late to join the rest of the squad is anathema. The clock is ticking yet Shaw, who may well be the United and England left-back for the next decade, remains courteous, asking politely to depart. It is a moment that captures the teenager’s rounded character and explains how a breathless past few months – a “whirlwind”, he says – has been taken in his stride.

To recap, Shaw made his England debut on 5 March last year, replacing his boyhood hero, Ashley Cole, at half-time in a 1-0 friendly win over Denmark at Wembley. Two months later, Roy Hodgson selected the 18-year-old for the World Cup at the same time as United began to firm up their interest in making him the most expensive teenager in history by paying Southampton £30m for his services.

Cole was the fall guy for Shaw in the squad for Brazil and the latter made an appearance in the final group game, the draw with Costa Rica, to become the second youngest England player to appear in a World Cup, behind Michael Owen. The transfer to United was sealed on 27 June and less than a month later Shaw was flying to Los Angeles to start a summer tour during which Van Gaal’s observation that he needed to be fitter was met by a response impressive enough for his manager to cite as an example to the squad.

And as if that was not enough, Shaw suffered a hamstring injury just before the season started. His debut did not come until 27 September, a 2-1 win over West Ham United at Old Trafford, and further niggling injuries have limited him to 11 appearances. But there have still been flashes of the quicksilver defending and searing attack play that makes Shaw such an asset.

Asked to look back at such an intense period, Shaw says: “It’s been a whirlwind, crazy. I didn’t really think of it like that, I sort of tend to focus on the football. It’s quite surreal how everything’s gone in the last six months – it was crazy. You don’t want to think too much of the past, you just want to focus on what’s coming up and the games coming thick and fast.”

How was Brazil 2014? “It was unbelievable,” Shaw says. “To go to the World Cup at that age was something amazing, and I learned a lot, especially with the likes of Bainesy [Leighton Baines]. I was always watching to see what he does, also in training as well – training with world-class players.”

Shaw is a keen observer. “Especially when you’re on the bench you want to see what the player in your position is doing. Luckily I had Baines – he’s one of the best left-backs around. So it was great to be able to watch him and have a conversation with him.”

Next came a realisation of the sheer size of the club he had joined during a tour that took in LA, Denver, Washington, Detroit and Miami. “You don’t actually know how big it is until you sign for it – around the whole world, the fan base and everything,” he says. “The teams we played as well – it was sort of weird playing the likes of Real Madrid and Inter and still being 18, it was very nice to be playing against [Cristiano] Ronaldo and Gareth Bale. That tour helped me a lot.”

Luke Shaw made his Manchester United debut against LA Galaxy in a pre-season tour to the US, and later played 45 minutes against Internazionale

Van Gaal’s comments about Shaw’s fitness came in late July before the third match against Internazionale. A day later, Shaw fronted up to admit the manager was correct, especially given the demands of Van Gaal’s new wing-back system. “I wanted to clear the air,” he says. “Maybe some people took it the wrong way – I think he was trying to help me out and it’s all over now. That was in pre-season and everything’s fine.”

Van Gaal has tinkered with United’s shape, often switching between a three-man defence and the traditional back four. Had Shaw played wing-back previously? “At Southampton we played with high full-backs, you could say that’s wing-back, but not particularly, no,” he says, then suggests he can adopt a more attacking role. “I’m always learning, especially with the different formations. It will help me in the long run to maybe become a forward player – on the wing or something.”

Asked about that record £30m transfer, Shaw shrugs. “I’m a player coming into Manchester United, I’ve got to adapt like every other, no matter how old I am,” he says. “If I was 28 or 19, it doesn’t matter – you’ve got to be able to come in here and play the way the manager wants and that’s it. My family brought me up very well. I’m not one to boast, I’m more relaxed and focused on what I need to do.”

Heidi Stone, the assistant head principal at Surrey’s Rydens Enterprise School, where Shaw went, describes him as “popular both with teachers and peers” and “quite humble and modest. You would never see a sign of arrogance.” Shaw is happy to hear this. “Especially from my teachers. I’m still quite close to some of them because I want to go and visit my old school – they’re part of the reason of why I am where I am now. If I didn’t have such a good background with them I might have been struggling,” he says.

His professional debut for Southampton, whom he joined as an eight-year-old, came at 16 at Millwall’s New Den on 28 January 2011, in an FA Cup tie. “I remember I came on at left-wing,” Shaw says of entering on 77 minutes for Jason Puncheon. “It was a horrible place, horrible to go to Millwall. I remember warming up and there were fans chanting stuff to you. It was very tough, and I was so tired even though I’d [only] been on for 20 minutes. I wasn’t used to that pace and it was a very physical battle because when you go to clubs like Millwall it’s not really a footballing game.”

By the time he made his Premier League debut, in November 2012 against Swansea City, he had already achieved something few England footballers have: winning a trophy for the national team, after the Under-16s won the 2011 Montaigu Tournament in France.

Shaw scored in the opening group match against Uruguay. “It was a penalty, I’m not sure if it went in – it hit the crossbar and went down to the line but I remember that because we won it, didn’t we?” he says.

United’s outside hopes of winning this season’s title took a tumble after Sunday’s 1-0 defeat by Southampton left them 12 points behind Chelsea. But he refuses to give up – next is Saturday’s trip to Queens Park Rangers and the challenge is to win every one of the last 17 matches.

Shaw says: “We’re always focusing on first. We wanted to win the league but obviously it was a massive setback from the defeat on Sunday. So we just need to keep going and take it game by game and hopefully pick up maximum points.”

• Luke Shaw supports the PlayStation Schools’ Cup, which has increased football tournaments for boys and girls across secondary school age categories, attracting close to 2,000 school entries and 100,000 participants. The grassroots programme will help nurture stars of the future, creating more opportunities for children to represent their school football team. Visit playstationschoolscup.com