The Blues won the FA Cup but finished the Premier League on fifth place and therefore missed out on the Champions League football.
"The Premier League over the last few years has become more competitive. There is a lot of big teams, a lot of replacements," Deco told Express Sport.
"I think we will see changing winners in the next years. Manchester City, they did something not normal in the Premier League.
"The way they play, Diego was very important for them. It was difficult to find a substitution. Morata is not the same, even Giroud is not the same. Diego was for me the perfect player for the way Conte played.
"I think they lost [Nemanja] Matic too, they had good midfielders but I think for me it was Diego Costa [leaving]."