United manager Ole Gunnar Solskjaer ditched his usual FA Cup keeper Sergio Romero to keep faith with De Gea – but the experienced Spain international produced two horrendous errors either side of half-time to gift Chelsea two goals.
De Gea made a flimsy attempt to deal with Olivier Giroud’s flick in first-half stoppage time then weakly fumbled Mason Mount’s tame 20-yard shot into the net moments after the restart.
Chelsea’s excellent performance fully merited this victory and United’s abject misery after a dreadful performance was complete when captain Harry Maguire diverted Marcos Alonso’s cross into his own net at the near post with 16 minutes left.
Frank Lampard’s side were in complete control from first to last as they comprehensively ended Manchester United’s 19-match unbeaten run in all competitions.
Bruno Fernandes pulled a goal back from the penalty spot late on after Callum Hudson-Odoi fouled Anthony Martial but it could not even be described as a consolation as Chelsea closed out the win.
Lampard is now in position to make his first season in charge a highly satisfactory one as they contemplate a place in the FA Cup final and lie third in the Premier League, in position to reach next season’s Champions League.
And no-one can question their right to this win as they dominated from start to finish, particularly in midfield, where they were able to subdue the influence of the talented Fernandes.
Once again 33-year-old Giroud demonstrated his influence, leading the line selflessly and being rewarded with that crucial first goal, even though it received a large helping hand from De Gea.
Chelsea were more energetic, dangerous and aggressive and while they were thankful to those mistakes from De Gea, the margin of victory was no more than they deserved.
Lampard’s side cut off any supply line to United’s forwards and it will be an intriguing final on 1 August back here at Wembley, when he pits his wits against another rookie manager in Arsenal’s Mikel Arteta.