Mancini says Chelsea's instability will hurt them.
He told the Guardian: "For me, Carlo (Ancelotti) was the strange one. Carlo is one of the best managers in the world for me. He won the league and the FA Cup and then they sacked him. It's difficult for a club that change every year, every two years.
"(Sir Alex] Ferguson's a totally different situation because he started to work for United in a different time. Now he's like a seat in the stadium, the grass on the pitch. He's part of United."