Man Utd will face Fenerbahce, Feyenoord and Zorya Luhansk in the Europa League, while Southampton are in a group with Inter Milan.
Inter Milan suffered a surprise 2-0 defeat to Chievo on the opening weekend of Serie A while Napoli also struggled.
Francesco Totti made his 600th Serie A appearance as Roma moved up to second spot with a 3-0 win over Chievo.
Mario Balotelli faces an uncertain future after Serie A side Sassuolo turned down the chance to sign him.
Juventus celebrated winning the Serie A title for the fifth year in a row by beating relegation-threatened Carpi 2-0.
Paulo Dybala scored twice as Juventus thrashed Lazio 3-0 to take what could prove to be a decisive lead in Serie A.