Barca extended their lead at the top as they moved to 30 points, six above Real and nine clear of Villarreal in third.
Lookman bagged a hat-trick as Atalanta won its first-ever European trophy last season.
England, meanwhile, are left heartbroken after suffering defeat in a second consecutive European Championship final.
Mainoo went on to play 32 times for United, scoring five goals.
The Spanish team fought their way back into the game from behind to secure a 2-1 win over France in Munich on Tuesday.
Napoli struggled to make an impact in the first 45 minutes, failing to register a shot.
