The Giallorossi agreed a deal, which including taxes, fees and performance-related bonuses comes up to 20m, with River Plate on Saturday.
"I'd say the negotiations for Lamela are closed. There are some marginal details, but the deal is in place," Sabatini told Sky Sport Italia.
"It is an important sum, but I think he's worth the money, because Lamela is a young player who is already at a high level and has a bright future ahead of him."
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