Tottenham spent a club record £100 million, including add-ons, to lure the 26-year-old from St James’ Park to North London.
Tonali is now expected to form a key part on Tottenham’s new midfielder, alongside £85 million man Mateus Fernandes.
De Zerbi 's side have also tied down Micky van de Ven, Pedro Porro, Ben Davies and Antonin Kinsky to new contracts as they seek to ensure another relegation battle doesn’t happen.
Alongside the players they already have, De Zerbi is convinced Tonali has all the leadership qualities needed to be a success at the club.
"If I really did convince him, then I can only be happy about that," De Zerbi told Sky Sport Italia.
"I told him the truth, because I always tell players what I really think. I believe he is a player who has always been a leader in every squad he has played in.
"So here, with a team that is in the process of reconstruction, he can have an important role.
"I was already watching Tonali when he was playing for Brescia, my hometown club and the team I support. At the age of 18, he already looked like a veteran.
"I think this is something that Tottenham and I can offer him, more than other clubs.
"This is my first 'real' season at Tottenham, the last was about survival, but this time we can start building something lasting in the most difficult league in Europe."