The Montreal Impact centre-back, 37, believes the power of La Liga, the Premier League and Bundesliga has been responsible for the departure of the best players from Italy's domestic competition and diminishing the strength of the 20 top-tier clubs.
But the lack of financial capital across much of Serie A hasn't prevented the Bianconeri from signing Tevez and Fernando Llorente and Nesta did concede the decision of Argentina striker was a coup for Serie A.
"There's no money in Italy at the moment and the best players go to play in other leagues - Spain, England, Germany. Italy's going down," the Montreal Impact centre-back told the BBC.
"It's a very good signing. A player like Tevez, I didn't think would play in Serie A now. Juventus did a good job."
The former Lazio star also praised former Milan boss Carlo Ancelotti, claiming the Italian was an ideal fit for Real Madrid after he replaced Jose Mourinho at Santiago Bernabeu on the back of two years with Paris Saint-Germain.
"He is the best coach for Madrid," Nesta said.
"Madrid is a difficult place to coach. There's a lot of pressure and Ancelotti is very good when he's under pressure.
"(Silvio) Berlusconi and (Adriano) Galliani, all the time, wanted the best for the team (at AC Milan). The pressure was very, very hard but Carlo Ancelotti, everyday, had a smile. He was perfect."
Nesta spent 10 years with the Rossoneri between 2002 and 2012, before leaving to join Montreal Impact in the MLS.