"He has characteristics that none of our other midfielders possess, especially playing between the lines," Sarri told reporters as his team prepared to face Shakhtar Donetsk in their opening Group F Champions League match.
"For me, he is irreplaceable, which is why I always substitute him in games because otherwise we wouldn't be able to give him a rest."
Napoli have won all five competitive matches this season - three in Serie A plus both legs of their Champions League playoff -- and Sarri said they had patched up their defence, often seen as a weak spot last season.
"We are more solid at the back which has helped us get through the difficult moments," he said. "Even so, we always aim to dominate the game from the start to finish.
"I wasn't happy with the last two games which we only won after moving up a gear near the end."
(Writing by Brian Homewood; Editing by Toby Davis)
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