He limped off in training on Wednesday and tests found a left thigh strain that is likely to keep him out for up to a month.
"It's terrible timing," Mazzarri told Tuttosport. "Just this week during the break for international duty we were going to increase his work rate in training.
"Diego is a player who changes the balance of a game. He is a top player, the kind that's difficult to find nowadays."
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