Leonardo Bonucci AC MilanGetty Images

Best defender in the world? What the hell has happened to Leonardo Bonucci?

It had started out as a joke.

Every time AC Milan sporting director Massimiliano Mirabelli sounded Alessandro Lucci out about one of his players during the summer, the agent would casually drop in the name of his most valuable client, Leonardo Bonucci.

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Mirabelli thought nothing of it until it became clear on July 12 that not only was the defender actually ready to leave Juventus, the Italian champions were actually prepared to sell the centre-piece of their brilliant backline.

"Bonucci's transfer wasn't planned but it was an opportunity we had to grab as it went past," Mirabelli told Rai Sport.

"Over the first few days, I didn't even tell [CEO Marco] Fassone; I thought Lucci was joking. Then, I saw the lad really did want to come to Milan and we worked out the negotiations very quickly."

Indeed, the deal was done inside 48 hours, with the Rossoneri agreeing to pay Juve €42 million for the services of a man also coveted by Chelsea and Manchester City.

Vincenzo Montella Leonardo Bonucci AC MilanGetty Images

Even Milan's players were stunned by Bonucci's sudden arrival from Turin and had no issue with the new signing being immediately installed as club captain.

"I didn't actually believe he'd come!" Giacomo Bonaventura enthused. "Leo is a winner, a pillar of the national team and his arrival is a fantastic thing.

"From what I know of him, his character will be very useful in the dressing room, which is coming off the back of some troubled years. He's destined to become a real point of reference."

The reverence of his new team-mates was understandable.

Since joining Juventus in 2010, Bonucci had developed into the finest defender in football, one of Pep Guardiola's "favourite ever players", a man who played a pivotal role in the Bianconeri's unprecedented run of six consecutive Scudetti and re-emergence as a major European force.

He was not just a central defender, he was central to how Juve played because of his wonderful passing skills. Bonucci was, thus, viewed as the major coup of Milan's summer spending spree, the one signing sure to succeed, a natural-born leader capable of turning losers into winners.

Now, though, some feel that the joke is on Milan.

Leonardo Bonucci AC Milan PSGetty Images

Bonucci was confirmed on Monday as one of the 30 nominees for this year's Ballon d'Or, but that news only served as an awkward reminder of just how dramatic a dip in form the Italy international has suffered since his switch to San Siro.

The best defender in the world suddenly looks like one of the worst in Serie A, prompting former Napoli and Udinese director Pierpaolo Marino to claim: "I think Milan already regret buying Bonucci.

"I think that if Massimiliano Mirabelli and Marco Fassone could go back in time, they'd spend that money on a top striker instead."

That is most certainly not the case, though.

Admittedly, the unplanned nature of Bonucci's arrival left Vincenzo Montella with an unexpected dilemma: should he change his formation to suit his most significant summer signing? The Rossoneri had last season played with a flat back-four, but Bonucci's preferred position was in the centre of a three-man defence.

Montella decided to change, meaning not only were Milan starting the season with 10 new players, they would also be doing so with a new formation. Therefore, the coach and his team were always going to require time to adapt. The surprise, though, is that Bonucci has looked so unsure of himself, even in his beloved back three.

There have been uncharacteristic lapses in concentration, inexplicable unforced errors and even his famed distribution has repeatedly betrayed him. He was meant to raise Milan's game; instead, he has dragged them down.

Leonardo Bonucci Montella AC Milan PSGetty Images

"He certainly knows he has to improve his performances, but he has been the first to admit it," Montella said before the international break. "But he is not a problem and he never will be. He is a resource for Milan and will be for many years to come.

"People seem to think everything is Bonucci's fault, but I know how to get the best out of him and his playing style."

It's time now for Montella to back up that claim, with Bonucci having returned to Milanello this week after two dismal displays for Italy, against Macedonia and Albania.

The Rossoneri sit seventh after seven rounds of Serie A action, having lost three games already, including their last two, away to Sampdoria and at home to Roma. A third consecutive defeat would be catastrophic for Montella's hopes of holding on to his job, as Milan's next opponents are city rivals Inter.

As a result, the coach desperately needs the real Bonucci to stand up at San Siro on Sunday evening. The defender's poor form has gone well beyond a joke at this stage.