Mancini defended SuperMario throughout his spell in England, but opted to accept Milan’s €20m offer in January.
“We wanted to keep him with us for many years, then Milan dived in and things changed,” Mancini told Radio2 in Italy.
“He is happy to have returned to Italy and will form the best strike partnership in Serie A with Stephan El Shaarawy.
“However, Mario must realise that the career of a football player is brief. Just look at Adriano, who was the best and in such a short period of time he disappeared.”