Sebastiano Esposito put away a second-half penalty which made him the youngest Inter Milan scorer since Mario Corso in 1958.
The Serie A match in the San Siro had been overshadowed by a racism storm after Italian sports daily Corriere dello Sport's controversial headline.
Juventus battled to a 1-1 stalemate at lowly Lecce, but the defending champions still stay top in Serie A as nearest rivals Inter Milan drew 2-2 with Parma.
Serie A: Romelu Lukaku and Lautaro Martinez each bagged a brace in a seven-goal thriller to keep Inter Milan second, a point behind Juventus.
Euro 2020: Romelu Lukaku took his record international tally past 50 goals as Belgium opened up an unassailable 11-point lead over third-placed Cyprus.
Kevin De Bruyne set up three goals inside the first 32 minutes before capping a brilliant individual display by rounding off the scoring.
Belgium's all-time top-scorer with 48 goals Romelu Lukaku has not played a single minute of Manchester United's pre-season campaign ahead of the move.
Manchester United who finished sixth in the Premier League last season have bolstered both attack and defence with the signing of two young talents Daniel James and Aaron Wan-Bissaka.
Paul Pogba, who has been linked with a move to Juventus and Real Madrid, starred in Manchester United's dominant first-half.
Manchester United have won all five matches since Ole Gunnar Solskjaer took the helm at Old Trafford following the sacking of Jose Mourinho.
Manchester United, last season's losing finalists, were 2-0 up before half-time, thanks to goals from Juan Mata and Romelu Lukaku.
Romelu Lukaku and Marcus Rashford were the goal scorers for Manchester United.
Manager Jose Mourinho made four changes to the side against Fulham after their draw with Arsenal in midweek.
Romelu Lukaku has failed to score for Manchester United since September 15.
Manchester United travel to Juventus for another date with former favourite son Cristiano Ronaldo, two weeks after they were outplayed at home.
Romelu Lukaku scored two goals as World Cup semi-finalists Belgium began their Nations League campaign with a comfortable 3-0 win away to Iceland on Tuesday.
Belgium showed their class and no hangover from their exertions in finishing third in Russia to warm-up for their Nations League opener against Iceland.
Lukaku's first-half double was enough for a well-earned three points as United bounced back from consecutive defeats to Brighton and Tottenham.