#4 Andrea Pirlo
Inter Milan allowed Pirlo to leave after 40 games for them. He joined their cross-town rivals AC Milan in 2001. What Inter did not realise was that they were not selling a youngster, but a player who would later become one of the best deep-lying midfielders of the modern football.
Pirlo went on to become one of the best midfielders of his generation winning six Serie A and two Champions Leauge with AC Milan and Juventus.
His vision and passing made him stand out. He orchestrated attacks from deep with outrageous long passes that reached its target with incredible precision. Not only was he known for his playmaking skills from deep, but he was also described as a silent leader who would do the talking with his feet.
His cool and calm attitude was also very rare as he once quoted, "I don't feel pressure ... I don't give a toss about it. I spent the afternoon of Sunday, 9 July 2006 in Berlin sleeping and playing the PlayStation. In the evening, I went out and won the World Cup."
Inter Milan had a decent midfield duo in Dejan Stankovi? and Esteban Cambiasso. The addition of Pirlo would have undoubtedly made them stronger.
The only condolences Inter Milan can have is the fact that their rivals AC Milan made the same mistake and allowed Pirlo to join Juventus for free and he went to win 4 more Serie A titles with the Old Lady.