​​​​​​​Luke Shaw had the world at his feet when he made his £30m move from Southampton to Manchester United in 2014, but three years on he can barely get a game and his future is uncertain. What's gone wrong? And is there a way back under Jose Mourinho?

Paul Pogba soaked up the adulation. Zlatan Ibrahimovic stopped for interview after interview. Even the usually deadpan Michael Carrick had a grin on his face. But as the Manchester United squad gleefully celebrated their EFL Cup triumph at Wembley, there was Luke Shaw, shuffling around the pitch with his hands deep in the pockets of a black overcoat.

In United's biggest game of the season and against his former club Southampton, Shaw had not even been named on the bench. It's become a familiar story. The 21-year-old's only appearance in the last three months came in an FA Cup fourth round win over Championship strugglers Wigan. Since then, he has only made one matchday squad out of seven.

It's a curious situation. Shaw struggled in his first season at United and suffered a broken leg at the start of his second, but in between the poor form and cruel injuries there were signs of progress. Roy Hodgson compared him to Ashley Cole days before that fateful lunge from PSV's Hector Moreno at the Philips Stadion, and he started this season as Jose Mourinho's first-choice left-back.

Two years after becoming the world's most expensive teenager, it seemed Shaw might finally become a key player and consistent starter. He was named in United's line-up in the Community Shield win over Leicester, and a few weeks later he was talking up his relationship with a manager who once tried to sign him for Chelsea.

Luke Shaw in action against Bournemouth at the start of the season

"We've had a little joke about it," he told The Guardian. "He's a cool manager. 'Why didn't you come?' he wanted to know. I just felt I had more opportunity of first-team football here. But now I'm with him and I'm really happy he's here. It hasn't been the best few years but all of a sudden it feels really good, really positive."

It's not difficult to pinpoint the moment things changed. It was mid-September and United were playing Watford at Vicarage Road when, with the scores level at 1-1, Shaw backed off Nordin Ambrabat in the build-up to the hosts' second goal. Mourinho might have taken issue with his team's failure to track Juan Zuniga, but Shaw bore the brunt of the criticism.

"The second goal was a mistake that goes against our plan and training because our intention was for their wing-backs to be pressed," said Mourinho. Shaw had been "25 metres away" from Amrabat, according to the United manager. "It is a tactical but also a mental attitude," he added.

Luke Shaw pictured in training for Manchester United at Carrington

The pair could be seen exchanging words as Shaw was substituted, and the incident evidently made a lasting impression on Mourinho. Shaw had started every one of United's five Premier League games at that point. He has only started one out of 21 since. After a 3-1 win over Swansea a month later, he was even accused of pulling out of United's squad on the morning of the game.

Has Mourinho made an unfair example of Shaw or is there something else there? The truth probably lies somewhere in between, but he isn't the first to have doubts. Louis van Gaal and Roy Hodgson voiced concerns over Shaw's fitness during his first year at Old Trafford, and Nigel Adkins, the man who gave him his debut at Southampton, was aware of the issues long before that.

"I'd only play him half an hour at a time in the Premier League," Adkins told Sky Sports in May 2015. "I was maybe getting some questions asked of me but we knew Luke couldn't play a full 90 minutes. He was still 17 and very young. He even struggled to play a full 90 minutes in the Under-18 team."

Adkins described Shaw's talent in glowing terms, but revealed he used to ask to train with Southampton's scholars instead of the first-team. He also recalled how the youngster needed special attention in all aspects of his development.

"We had to spend a lot of time looking after Luke because he was very young and there were the off-field things that go with that," he added. "We had to put a special group together just to help him develop his all-round lifestyle and give him an opportunity to deal with the situations that arose."

Mauricio Pochettino, Adkins' successor at Southampton, offered a similar appraisal in December 2013. "Luke is still learning how to be an experienced player and it is taking him time to mature," he said. "He is growing, maturing and needs to develop psychologically, mentally and tactically."

In hindsight, it's clear why Shaw found the step up difficult at Old Trafford. His mentality is a recurrent theme among his former coaches, and his past also provides clues as to why he might not have clicked with his current manager. Shaw responds best to an arm around the shoulder, but Mourinho is an uncompromising boss who likes tough characters who can take responsibility.

Luke Shaw vies for possession against his old club Southampton

"I am playing with Daley Blind, with Marcos Rojo, with Darmian and all of them are playing the way I like a full-back to play," said Mourinho last week. "Luke has to wait for his chance and work better knowing that I give nothing for free. When I give something to the players it's expensive for them, they have to work really hard every day."

Shaw can take inspiration from Henrikh Mkhitaryan, who fought his way back into the team after failing to impress Mourinho, but the United boss is currently so unconvinced that he's willing to go without a natural left-back. For a manager with a preference for specialists, it says a lot.

It also highlights the size of the task facing Shaw. "In this moment he is behind the others," added Mourinho. "Potentially he has many things that I like but potential is one thing, expressing all the qualities that I like a player to express on the pitch is another." The message is clear. If Shaw wants to join scenes of celebration like those at Wembley, it's down to him to start meeting his manager's demands.