Less than a year ago, Renato Sanches was the toast of Europe. On Sunday he will make his Swansea debut. So what went wrong for him at Bayern Munich, and how good can football’s captive prince still become?

Football has always tended to eat its young. Although not, perhaps, with such gleeful, richly rewarded haste as this. The ballad of Renato Sanches, European football’s great invisible wonder kid, is set to enter its latest phase on Sunday afternoon as Swansea City take on Newcastle United in their first game since Sanches joined on loan from the dingiest corners of Bayern Munich’s Säbener Strasse training complex.

From obscurity to celebrity to entropy to the Liberty. And all in less than two years. It is now 23 months since Sanches made his first-team debut for Benfica as a thrillingly ragged, thrillingly high-grade 18-year-old from the tough side of Lisbon. The timeline of that career parabola bears repeating.

Nine months on from his debut, a previously ailing Benfica won the league, driven on by some extraordinary, bullocking performances that earned Sanches a (then) huge-money transfer to Bayern Munich. In June he scored a brilliant goal in the Euro 2016 quarter-final against Poland, was voted the best young player of the tournament and stood pretty much unchallenged as the most eagerly coveted teenage talent in world football.

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The same month Lothar Matthäus compared his influence – favourably – to that of Cristiano Ronaldo. Pep Guardiola called Sanches “by far and away one of the best young players in Europe”. In October, 12 months on from his debut, he was voted the Golden Boy of European football. At which point, it seems fair to say you might have got fairly long odds on Sanches preparing less than a year later to make his Swansea debut as a summer replacement for Jack Cork.

But then the speed with which Sanches has become cold product is just as striking. Aged 20 he has to date made 42 starts in club football. This season he has seven minutes of proper football to his name, at the tail end of a year that saw him register no goals and no assists for Bayern Munich’s steamrollering Bundesliga champs.

At the end of which he will make his debut in English football not for Manchester United, who tried to sign him at least twice, nor for Chelsea or Liverpool, who both had a shot this summer, but for a team who are among the favourites to go down, and who have never paid more than a fifth of his market price for a midfielder.

It is a career-fade all the more baffling given the basic nature of Sanches’s talent. This is not some flickering vision of fragile promise, but a player who announced himself as a ruggedly assertive presence, able to shoot, tackle and pass, to work the angles and rhythms of central midfield. And yet a very obvious brittleness remains. In June Sanches played 40 minutes for Portugal Under-21s against Slovakia having not been seen for three months and produced a performance of grippingly amnesiac midfield brilliance, all clumsy, shanked passes, returning touch and at the end the most beautiful pass for Portugal’s second goal.

Paul Clement, whose contacts at Bayern helped secure the deal, is well aware of the sensitivity of his situation. “The fact that I have already worked hard to get him here means he can come here with confidence. He’s going to a club that really wanted him and a coach that really wanted him,” Clement said on Friday morning. “I think there will have to be a certain amount of patience we show with him, given what’s happened to him in the last season. But he’s going to be hungry to do really well this season, so that he can perform at a much higher level next season in European football.”

All of which does seem some way off for a player who arrived in Swansea on Thursday night and who will have had just two training sessions before the Newcastle game. What is certain is the reassembling of Renato Sanches will take time. And that the question of how good he can be will only begin to be answered when the reality of just how bad he was at Bayern is fully digested.

Mark Lovell is the Bayern Munich correspondent for ESPN and watched Sanches close-up last season at the Allianz Arena. “There were too many headless chicken, frenzied running about-type performances,” he says. “Sanches was the worst passer in the Bayern squad last season. He was too keen to impress in his cameos and ended up giving the ball away cheaply, which was really noticeable in the first season after Pep [Guardiola]. I think expectations got the better of him.

“If Swansea are more realistic, with the message that he is far from a world-beater aged 20, then he might do quite well. And despite me only pointing out his failings, Bayern haven’t given up on him and want him back next season.”

One problem for Sanches was that he spoke no German, just as he speaks no English or Welsh now. Swansea’s Portuguese coach Nelson Jardim will act as translator but the awkwardness of Sanches’s public appearances only emphasises the sense of slight immaturity.

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Sanches did not want to leave Benfica but the club could hardly refuse the greatest deal in its history: a fee that could rise to €45m for a player who cost €750 plus a promise of 25 footballs for his local club, Aguias. And perhaps one issue with Sanches is that a strength from his development years has become a weakness in the glare of stardom. He is a genuine favela player, a kid from a tough upbringing who played his junior years in local teams rather than in an academy and had to be persuaded to leave Aguias for the lush pastures of Benfica’s training complex across the river Tagus.

The simplicity of his style, not to mention his prodigious physical power and stamina, were wonderful assets in that first year and right on into the European Championship, where Sanches kept the ball without fear in the centre of the pitch. That confidence is vital to his game. At his best Sanches loves to take risks, to be bold in his passing, to produce moments that can decide a game.

In Portugal, teams took to pressing him with two or three players when he picked up possession, wary of his ability to carry the ball 30 yards or so with startling ease. Sanches, who came from nowhere, and still has not really done much, seemed to be learning on his feet, growing with every game, trajectory set towards the sky.

All of which makes his move from Benfica look more destructive, a case of instant gain above all else, stocks being cashed in as the price rose. Benfica were desperate to keep him, with a year in the Champions League to look forward to. The agent Jorge Mendes pushed the move through. At the end of which nobody will ever know where Sanches might have been now had his development not been interrupted.

It is a common theme in modern football, the insistence on monetising prodigious ability as soon as it becomes evident. Anthony Martial springs to mind. Time will tell, but Kylian Mbappé certainly seemed to be in just the right place last season.

“There are examples of others where it has worked fine,” Clement said. “Rooney to United worked. Ronaldo to United. We are going to try and make sure this one works well. It’s for one year and we have to accelerate it quickly to work. It will certainly be a strange new environment for him, but he’ll play a lot and will suit Swansea’s style of play.”

And this really does seem to be the key. Whether in Bavaria, south Wales or north of the Tagus, Renato, the captive prince, just needs to play.