The decline started after 2015. Allegri masked our problems in 2017 imo and they were brutally exposed in that final.
What made Allegri great above all was his winning mentality.
People tend to forget how useless we've been in Europe before and after Allegri. Getting knocked out to the likes of Lyon, Benfica, Galatasary and now 10 men Porto. It was Allegri's winning mentality that won us over Real Madrid, Barcelona, Man City and Tottenham (yes Tottenham, our record against English teams was pathetic back then).
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Sale of Vidal imo