Tomori joins on loan with option to buy for a reported €30m.
Massara said, “Whenever we sign a player on loan, we always try to get an option to buy. We had to fight pretty hard with Chelsea to get that option on Tomori.
“We believe he is versatile and suits our style of football, so he brings characteristics in his aggression and pace that some of our other players in those roles don't have in quite the same way.
“We consider our defence complete with these players and trust we can continue well with this squad. It's tough from tonight, of course, as Atalanta are capable of troubling any opponent."