Kevin-Prince Boateng was clearly riled. He had clashed with Michael Ballack a few moments before and they had exchanged words. Then, in the 35th minute of the 2010 FA Cup final between Portsmouth and Chelsea, the Pompey midfielder ran 15 yards and clattered into Ballack from behind. The Germany captain was left on the floor, unable to continue, his World Cup dreams for ever in tatters. He never played for Germany again.

The outrage back home was heartfelt and in that moment it was completely unthinkable that Boateng would, three years later, be welcomed back to the country of his birth as the signing of the summer. But that is what happened.

Schalke, who are playing Chelsea in the Champions League on Tuesday night, announced on 30 August that they had signed the 26-year-old Ghana international from Milan for €12m (£10.1m). No one had seen that transfer coming. Six years since leaving Hertha Berlin under a cloud – the German papers had nicknamed him "RAMBOateng" and "Ghetto-kid" – he had returned a world-famous footballer. He once told the German football magazine 11Freunde that he wanted to be "more than just a footballer" and during his time at Milan he became just that.

To focus only on the moment, last January, when he had had enough of the racist abuse he was subjected to during a friendly, would be unfair because he has developed into a marvellous footballer. However, it was his stand against racism that suddenly catapulted him into the world spotlight.

His Milan team-mates followed him off the pitch and his actions caused a huge debate in Italy. The Italian league introduced new, tougher sanctions on racism, on and off the pitch, and in August a fifth-division player was suspended for 10 games after racially abusing an opponent. That was two more matches than Luis Suárez and six more than John Terry.

His profile soared and Nike ran an advert with the slogan: "I would do it again. And again. And again". Boateng, two months later, told CNN: "I am sad and angry that I'm the one that has had to take action. I saw massive support from England and players such as Rio Ferdinand and Patrick Vieira, and I want to say thank you. I think we should not accept and tolerate it anywhere, in any game. Even if it's a World Cup final, if it's a Champions League final. For me, honestly, I would do it even in the Champions League final."

There have been other headlines, too. After Milan won the scudetto in 2011 he dressed as Michael Jackson and performed a moonwalk – pretty well, it has to be said. Since that moment, his team-mates referred to him as the "pop star". Then there was the time when his girlfriend, Melissa Satta, told the Italian press how often they had sex every week and some papers linked that to the many muscle injures he had that season.

But Boateng is first and foremost a footballer and will be key to Schalke's hopes of progressing in the Champions League. He has struggled with a knee injury recently but Schalke, who are three points ahead of Chelsea at the top of Group E, are confident their No10 will play on Tuesday.

That Boateng got as far as playing even one Champions League match is a tremendous achievement. He grew up in Berlin's Wedding district and played with his two brothers – Jérôme, who is now a Germany international playing for Bayern Munich, and George – on a concrete court nearby. "In Wedding you become a drug dealer, a gangster or a footballer," he once said.

He joined Hertha Berlin as a seven-year-old and progressed through the youth teams before making his first-team debut as an 18-year-old in August 2005. Two years later, however, the cash-strapped club decided to sell him to Tottenham Hotspur for about £5.4m.

Immediately he was told by Martin Jol that he did not feature in the manager's plans. It was a ploy to force Boateng to work harder but it backfired. He lost interest and started spending his money in bars and on expensive cars.

"I was just very unhappy," he told 11Freunde. "Short-term I didn't mind whether I was playing for the first team or for the reserves in front of three people, but after four weeks or so the car thing became boring and then all the everyday problems came back – with a vengeance." He split up with his pregnant wife, Jenny, and was sent out on loan to Dortmund. There, he rediscovered his form and there is a lovely quote from Jürgen Klopp, saying that Boateng is such a good player because he always has "100 solutions and ideas" for a situation on the pitch.

Sadly for Boateng, though, Dortmund missed out on the Europa League and could not afford to buy him from Spurs. He joined Portsmouth first on loan and then permanently, where he prospered under Avram Grant and Paul Hart, with the latter telling him before he went out on the pitch: "Go out and play, young man and do what you want but please, please make sure you don't get sent off."

Boateng revelled with the freeness of his new role and the added responsibility. It was not, however, until he met the adviser Roger Wittmann that his career properly took off. Wittmann told him to get a personal trainer and in a few months he had lost 12 kilos.

Soon after that horrendous tackle on Ballack he joined Milan, where he quickly went from a decent player to become a really good one. Simultaneously, his off-pitch profile continued to grow. Finally, he had become "more than a footballer" and on Tuesday, injury permitting, he will aim to show the British public what became of the player who failed to make it at Tottenham and was relegated with Portsmouth.