The 24-year-old full-back will pen a deal that will last until the summer of 2029.
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It is believed that Everton have secured a substanstial sell-on clause, and will now chase Fulham's Kenny Tete on deadline day in their continued hunt for a new full-back.
Patterson has made 60 appearances across five seasons for the Toffees, with his time at the club being ruined by injuries.
The defender will become the next Scot to venture to Italy, and is set to join up with his international team mate Che Adams.