In the final days of the summer transfer window, Roma approached Internazionale about the possibility of signing Fredy Guarin. Nerazzurri sporting director Piero Ausilio's response was as swift as it was unequivocal: Guarin is unsellable.

That was hugely significant, given Inter was under pressure to offload players due to a summer spending spree coupled with Financial Fair Play regulations. Mateo Kovacic was allowed to join Real Madrid, while Hernanes was sold to Serie A champion Juventus on deadline day - yet Inter refused to even entertain the idea of cashing in on Guarin.

It was just as well, really, as the decision to sell Guarin to Juve in January of last year prompted such passionate protests among the Inter faithful that club owner Erick Thohir performed a last-minute u-turn and vetoed a deal that would have seen Mirko Vucinic move in the opposite direction.

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On Sunday evening at San Siro, Guarin proved just why he remains such a popular figure among Interisti despite his often inconsistent form, with the 29-year-old settling the Derby della Madonnina against AC Milan with a wonderfully measured left-footed strike from outside the area, after a terrific surge from midfield.

It was a strike of great quality in a game characterized by poor finishing. Both sides have spent huge amounts of money strengthening their respective attacks this summer but a number of clear-cut chances were squandered at San Siro.

Luiz Adriano missed a one-on-one inside the opening two minutes, while Mauro Icardi surprisingly botched an excellent opportunity to mark his first derby as captain (Inter's youngest skipper in a game against Milan for 50 years) with a goal just before the break.

Milan splashed a total of $43 million on Luiz Adriano and Carlos Bacca and they combined well throughout, confirming the early indications that this is a partnership of real potential.

However, the derby suggested that the loan signing of Mario Balotelli could well prove to be Milan's most inspired move of the summer. The Italy international, of course, is coming off the back of a disastrous season with Liverpool and question marks remain about his temperament – hence the insertion of a number of behavioral clauses in his contract. As Rossoneri CEO Adriano Galliani has repeatedly stated, the success of Balotelli's seconds spell with Milan "depends on Mario."

Encouragingly, he looked more determined than ever during a very exciting cameo against Inter. Of course, that is unsurprising, given the magnitude of the game and his past association with the Nerazzurri. Still, Balotelli created more havoc during his 28 minutes on the field than any other attacker on show.

Of course, Super Mario specializes in disruption - he leaves a trail of chaos wherever he goes - but he created all of the right kind of problems in the derby, with Inter struggling to deal with him. Indeed, only the post denied Balotelli what would have been a deserved equaliser.

Milan boss Sinisa Mihajlovic reiterated on the eve of the derby that Balotelli was still not in perfect condition; that he still had some body fat to lose - hardly surprising, given that this was his first competitive outing since April.

However, on the evidence on Sunday night, when he is fully fit, Balotelli is going to be a scary prospect once again, with his dynamic display underlining precisely why Inter boss Roberto Mancini considered re-signing the 25-year-old during the summer.

The result may been negative but Balotelli's contribution was undoubtedly the big positive. This was a very encouraging start to his second spell at Milan. Now he has to build on it. As ever, it all depends on Mario.

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