Manchester City completed the club-record £125m signing from Chelsea on deadline day, reuniting Fernández with the manager who worked with him — and made him captain — at Stamford Bridge.
Maresca confirmed the Argentina international is fit and available to make his debut against newly promoted Coventry City at the Etihad on Saturday.
"Enzo is ready, yes," Maresca said. "I know Enzo quite well and I know what he can bring to the team. He is a winner — a winner's mentality and knows how to win. The World Cup, Club World Cup... the main thing is he's a winner."
The move generated genuine excitement inside City's dressing room in the days before it was completed, according to Maresca.
"When you belong to a squad and another winning player arrives, they are always happy," he said. "I can promise you that in the last four or five days, four or five players from the squad were asking if he is coming or not, because they want him to be with us. It's very important that he is with us and we are 100% happy with that."
Despite handing Fernández the captaincy at Chelsea, Maresca confirmed Rúben Dias will continue as City's captain this season, with Erling Haaland, Marc Guéhi and Gianluigi Donnarumma completing the leadership group following Bernardo Silva's departure over the summer.
The switch comes after Fernández used a City in-house interview to say he had always "dreamed of being at a club as big as City," remarks widely read as a parting shot at Chelsea, where his relationship with the hierarchy had cooled since the turn of the year.
Outgoing head coach Xabi Alonso struck a more measured tone on the exit: "In football many things happen. With Enzo we have had conversations and always with respect. What he has done for the club has been great, but now it's time to move forward. We have players with quality in every position and now we are ready to go."
City's summer spending reached a club-record £458m, with £327m of that committed to a rebuilt midfield featuring Fernández, Elliot Anderson and Ayyoub Bouaddi. Wingers Iliman Ndiaye and Allan also arrived, along with back-up goalkeeper Gerónimo Rulli, while more than £300m was recouped through outgoing sales.
"I am very happy for the effort from the ownership, [chairman] Khaldoon [Al Mubarak] and [director of football] Hugo [Viana]," Maresca said. "In the last days, Iliman and Enzo arrived, and now the squad is complete — it's finished. Now it is time to work."