The Blues made a promising start to life under Lampard as the west London side hit the woodwork twice in the first half at Old Trafford.
However, Marcus Rashford broke the deadlock for the hosts from the penalty spot after Kurt Zouma brought down the England striker inside the area.
Chelsea FC were guilty of wasting chances in the first half and Manchester United made the visitors pay after the interval.
Rashford, Anthony Martial and Daniel James all got on the score-sheet to secure a resounding victory for Ole Gunnar Solskjaer’s side.
Lampard opted to start Tammy Abraham and Mason Mount ahead of players such as Olivier Giroud and N’Golo Kante at Old Trafford.
But the Chelsea FC manager defended his team selection in the aftermath of the one-sided rout.
“I can’t drag people out of the medical room to play, whether they are experienced or not,” Lampard is quoted as saying by Sky Sports.
“These are the players that would play today, and we were clearly the better team for 45 to maybe 60 minutes.
“What was evident is that we made four individual errors that led to goals out of their five shots, there’s the harsh reality for us.
“If we were more clinical, had a stroke of luck and went in at half-time two or three goals up, the game would be completely different.”
Chelsea FC appointed Lampard as their new manager during the summer after Maurizio Sarri left to take over the reins at Serie A champions Juventus.
The Blues finished in third place in the Premier League table last term and won the Europa League after a 4-1 win over Arsenal.
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