The former Roma player will add width to the Reds' attack, providing support for Roberto Firmino in an advanced forward position.
Philippe Coutinho, who recently handed in a transfer request amid interest from Barcelona, is out injured.
Hosts Watford have handed a debut to Nathaniel Chalobah, a summer arrival from Chelsea, starting him at the heart of a 4-3-3 formation.
Watford: Gomes; Janmaat, Kaboul, Holebas, Britos; Amrabat, Cleverley, Chalobah; Doucoure, Pereyra, Okaka
Subs: Pantilimon, Prodl, Femenia, Watson, Capoue, Richarlison, Gray
Liverpool: Mignolet; Alexander-Arnold, Lovren, Matip, Moreno; Henderson, Can, Wijnaldum; Salah, Mane, Firmino
Subs: Karius, Klavan, Gomez, Milner, Grujic, Solanke, Origi
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