August 15, 2016, was a day of high emotion for Alvaro Morata. Surrounded by his close family and friends, flanked by Real Madrid president Florentino Pérez, Morata was returning to the club where it all began. “You've come home,” Pérez told him. “And it's something that you have earned with your talent, strength and ability to sacrifice.”

After Pérez spoke, Morata stepped up to the podium to say a few words. It was all too much. As a boy in the back seat of his mother’s car, whenever they stopped at the traffic lights outside the Bernabéu, he would push his face up against the window, look up at the giant stadium and declare that one day he would play for Real Madrid.

Having already left once, and spent two years at Juventus, everything now seemed to be falling into place for this Madrid native. “Here I am again,” he said through the rising tears. “To give my everything, as I’ve always done at the Bernabéu, and try to stay here as long as possible.”

Eleven months later, Real sold him to Chelsea.

If Morata’s second return to Madrid was drenched in pathos, his second departure was anything but. “Goodbye without tears,” declared one prominent Madrid blog. The loss of a rare home-grown player was set against the stark reality that for a club that had just won the La Liga/Champions League double, and possessed one of the most feared forward lines in the history of football, £58 million was not a bad price for what was essentially a back-up striker.

Morata prepares at Cobham for Chelsea's match against Arsenal on Sunday credit: Darren Walsh/Chelsea FC via Getty Images

And so the split was, if not acrimonious, then hardly begrudging either. Off the pitch, Morata had been angling for a move for some months. His girlfriend Alice had already been house-hunting in London. On the pitch, meanwhile, it had become manifestly clear that Zinedine Zidane did not trust Morata in the big games. He started just one of their 13 Champions League fixtures, coming on in the 89th minute of the final to earn a barely-merited winner’s medal.

All this fed into one of the perceptions that has followed Morata throughout his career: that he was technically excellent, but lacking that final ingredient, the spark of fury that turns the great players into immortals. That he was perhaps just a little too nice. Gianluigi Buffon, his team-mate at Juventus, said Morata had the potential for greatness, “if only he could get over his mental hang-ups”. “Sometimes in football,” said former team-mate Diego Mariño in an interview with The Guardian earlier this year, “you have to be more of a b-----d. And he’s not.”

Morata scores with a header against Leicester and slides in celebration credit: Craig Mercer - CameraSport via Getty Images

What makes all of this not only highly interesting, but highly relevant, is the man he is replacing at Chelsea. Whether it is the sneaky kick in the shin, a surreptitious shove in the area or a bitter and protracted stand-off with his own club, nobody would ever accuse Diego Costa of needing to be more of a b-----d. On the face of things, Morata is a like-for-like swap. Under the bonnet, it is difficult to imagine two more different people.

For one thing, there is the background. In contrast to the resoundingly blue-collar Costa, Morata is the sensitive, worldly, well-rounded, middle-class boy. The son of a former commercial director at Spain’s biggest radio network, Morata turned down a potential tennis career in order to take his chances in the shark tank of the Real Madrid academy. That he made it clearly suggests he is made of more than simply talent. But as his father put it, to get the best out of Morata you need to show him a little tenderness. “He’s a man who has to feel at ease,” he said. “Like every human being, he has to feel loved.”

No one would ever say Diego Costa 'needed to be a bit more of a b------' credit: Mike Egerton/PA

On one level, there is something refreshing about a player prepared to brush against the grain of football’s machismo culture. There are certainly not many players who would have been prepared to admit that a poor run of form at Juventus in 2015 was the result of breaking up with his girlfriend, a former Miss Spain. Similarly, he credits his new partner Alice, a model who he met on Instagram, with helping him rediscover happiness.

Morata is certainly not short of love at the moment. He and Alice married over the summer, and since moving to England have eschewed the leafy suburbs of Surrey in favour of a city life, taking up residence in a luxury riverside apartment in Battersea. On the pitch, he has started strongly, with three headed goals in his first four games. And yet, as Chelsea prepare to host Arsenal on Sunday, the shadow of Costa lingers still, even as Chelsea’s poet-in-exile continues to sits on his sofa in Lagarto watching daytime television.

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Arsenal are arguably the team Costa loved riling more than any other. And while Morata may easily replace Costa’s goals, what remains to be seen is whether he will be able to replace his relentless aggression. Chelsea’s last two titles, under Jose Mourinho and Antonio Conte, were built to a large extent on Costa’s dirty work up front: chasing the ball down into the corners, holding it up to occupy defenders. Costa epitomised a team that was not afraid to win with a snarl.

Technically as well as temperamentally, Morata is a different sort of striker: a space-finder, a fine header of the ball who does not mind contact but would prefer to avoid it. He is also still finding his way. “English football is different to Italy and Spain,” he said this week. “You don’t have that much time to calm down or relax, because all the time the ball is on fire.” And so here is the rub: can Morata, the nice boy from the comfortable background, summon that same simmering anger that turned Costa into one of the Premier League’s least-liked opponents? And if not, how do Chelsea adapt?

“Don’t forget Alvaro is very young,” said Conte on Friday. “I think he can improve a lot. We are trying to work very hard with him, to adapt him into our style, our idea of football. For me, the striker is very important, very important. But he is a good guy and he wants to improve a lot. He wants to become one of the best.”

In a way, you could describe Morata as an incurable romantic. His Instagram feed is one long stream of smushy paeans to his new wife. He laments the over-commercialisation of the game in interviews. His tearful return to Madrid was no doubt driven by sentiment as much as anything else.

And yet, perhaps his ultimate failure to establish himself there can fuel the next chapter in his career. What happens when the best club in the world, the club you call home, lets you go? What sort of scar does that leave? And how far will you go to avenge it? Chelsea fans, you suspect, are dying to find out.