The midfielder spent last season on-loan with the Sardinians.
Giulini said, "The lad is in love with Cagliari for many reasons and would love to play here, but being a professional, he wants to challenge himself in Milan and probably show Conte he's not inferior to the other midfielders."
He also remarked: "One of the first things the coach (Eusebio Di Francesco) asked us was to work with Nainggolan again," Giulini told reporters including Sportitalia.
“As you know, it's Inter that must decide and this is not one of those years when we can afford to make certain offers to purchase players. We'll wait and see how the transfer market evolves.
“Obviously, we'd all love to bring him back to Cagliari in the right conditions, but we cannot afford to purchase him outright."