Spinazzola had agreed a four-year contract, with the club shelling out 29.5 million euros for the 26-year-old, Roma said in a statement. Juventus had paid 22 million euros for Pellegrini.
Roma also confirmed that Greek international Kostas Manolas had moved to Napoli for a fee of 36 million euros after spending five seasons in the Italian capital.
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Roma Chief Executive Guido Fienga had no doubts Spinazzola would settle quickly.
"Leonardo is a young player that has already acquired a lot of valuable experience in Italian football. I am certain that he will settle in here very quickly and will fit in well with the style our new coach implements," he said.
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