The Valencia striker signed for a €2m loan with obligation to buy at the end of the season for another €12m.
"It was a very good transfer campaign, I'm content," Ferrero told RMC Sport.
“There was a very bad agent, and I don't like bad people. I'm talking about the lawyer [Beppe] Bozzo [an advisor to Zaza, whose father is his agent] but I wish Zaza the best.
“As for Bozzo, I'd like to remind him that bad actions always come back to bite you."