The summer arrival from Manchester United has been superb in his first season in Serie A.
Manna recalled to Sky Italia: "We worked on players who were functional to our idea and who could raise the level of the team. McTominay played in a great championship and in a glorious team.
"It is a negotiation born a long time ago, then when we understood that there was an opportunity to take him we were determined, lucky and good at making it happen."
Manna also spoke of Khvicha Kvaratskhelia's January sale to PSG.
He said, "It certainly leaves a cloud of regret, losing an important player in progress is never easy. We talked about it a lot, it was also all very exploited in my opinion.
"This leaves me with the desire to always be ready to make up for things we can't control, but we are sure we did the right thing."