The midfielder's lack of mobility, tactical inflexibility and a year mostly spent on the sidelines all combined to convince those in power at Milan that Pirlo, good though he was, was great no longer. The Rossoneri had become a byword for past-their-best stars in recent years, and they didn't want another highly-paid veteran on the books. It was time for change.
For Pirlo, too, a change was needed. After a decade at Milan, the World Cup winner wanted a new challenge and an environment in which he was fully appreciated.
The 33-year-old is a class act, and he kept his feelings private, but it must have irked him to no end to find that after so many years of service, he was viewed simply as an immobile passer, chained to one position. Because after all, not all immobile passers are created equally.
Pirlo certainly lacks pace, the ability to run up and down for 90 minutes and the bulk to provide stout defensive cover in front of the back four. But then, as Pirlo's former teammate Zlatan Ibrahimovic once sagely pointed out, you don't buy a Ferrari to drive it like a Fiat.
The Italian's passing range is unsurpassed. His ability to change the shape of a game with one ball unequalled. Pirlo is the pendulum that keeps the rest of the players ticking, the conductor who controls the direction and the pace of almost everything that happens around him. He doesn't have to run around; he just gets everyone else moving instead.
His intelligence is something that Allegri shouldn't have underrated, either. On the face of it, an old player leaving the league champions for a side that finished 7th looks like fairly obvious evidence of a career in decline, but Pirlo could see the wind was changing.
He moved to Turin because he knew it was there, and not Milan, where he could continue to prosper. But there was another, earlier option on the table for the Italian—a move to London. So why didn't he take it, and what might have happened if he did?
It's hard to say why Pirlo didn't join his former coach and good friend Ancelotti at Chelsea. The pair are the perfect fit in terms of tactics, and no player was more important to all of Ancelotti's success at the San Siro. In many ways he's an atypical player in the modern game, and Ancelotti would have struggled to recreate his style at Stamford Bridge without him. The temptation must have been there, knowing that he'd be so integral to such a high-profile, success-driven side.
According to his autobiography—summarised here by the Metro—Pirlo pulled out of the move when Milan president Silvio Berlusconi intervened. Similar situations occurred with Real Madrid and Barcelona, too.
Had he chosen Chelsea, things would have been very different for Ancelotti at least. While the Italian's squad lacked little in terms of physicality, grit and determination, it was a little light on ideas. Pirlo would have been the bright spark that could have got everything moving in that difficult second season.
He might have even got Fernando Torres scoring. The Spaniard would surely have appreciated playing ahead of a deep-lying playmaker again, having gotten so used to Xabi Alonso at Liverpool and for Spain. Pirlo's passing combined with Frank Lampard's runs might have given the troubled Torres a familiar reference point.
Three managers later, with a fourth on the way, and Chelsea are still looking for that ideas man, at least if reports like the recent link to the aforementioned Alonso are to be believed (here, via the Independent).
Had they not secured his signature two summers ago, Juventus might now be in the same boat. Pirlo has become absolutely indispensable to Antonio Conte. The midfielder is a constant presence in the starting XI for the back-to-back Italian champions, and though he's only been in Turin for two years, he's already become on of the side's most iconic players.
Arturo Vidal and Claudio Marchisio are two of the finest central players in Europe right now, but the pair look a lot better when Pirlo's in with them pulling the strings. And you'd have to say that Juventus' front line, the only real weakness in their armoury, would struggle for goals without his passes and quick thinking.
Tempting to think what might have been then had he moved to SW6. What he might have inspired at Stamford Bridge alongside Lampard and behind the likes of Juan Mata and Torres or Didier Drogba. What other trophies could have been won in London; and what would have been lost back in Italy?