The Spanish international, who was joined by United instead of Sergio Romero, who played previous rounds of the cup, suffered a nightmare in the semifinals. His first mistake came in the 11th minute of the first half of stoppage time as he allowed Oliver Giroud’s strike to beat him in close proximity to Chelsea’s 1-0 lead.
The worst was to follow almost immediately at the start of the second as the excellent Mason Mount explodes forward and shoots from far away, seeing his effort under De Gea’s body pressure and in the goal.
De Gea brilliantly saved Giro’s second goal before Marcus Alonso’s cross was deflected by Harry Maguire, who was beaten again at his close position, in the 74th minute.
United, who entered the semi-finals of their back in 19 games without defeat, scored a late consolation as Bruno Fernandez scored from a penalty kick after Calum Hudson-Odoy’s superb challenge over Anthony Martial.
But they could have little complaints about the result with Chelsea Frank Lampard enjoying the best play from the start with Reis James being forced to de Gea in an early rescue before Alonso lost a golden opportunity.
A shocking collision between Maguire and fellow defender Eric Bailey resulted in a significant delay in work, as the Ivory Coast defender eventually extended with a head injury.
The accident may have bothered United, but de Gea was expected to be better with the inaugural than Giroud, his 16th goal in the FA Cup in seven years of English football for former club Arsenal and Chelsea and now in the fifth final.
“Okay, well, I told my kids in the dressing room,” Giro told BBC Sport, “I would love to win another one, it will be my fifth.”
“It’s another final, another big game against Arsenal, so it’s special for me, but we’ve put it on the side and we’re very happy with the team’s performance today.”
De Jia’s tragic mistake of a second from Mount toppled the padding outside United as director Olli Gunnar Solksjer turned to alternatives to Paul Pogba and teen revelation Mason Greenwood.
Instead, the third came when Maguire stretched out to block Alonso’s bid from reaching Antonio Rudiger, as the ball quickly entered the net from De Gea’s knee.
Lampard is now looking to his side by giving him his first silver as manager in the final at Wembley on August 1.
“I hope so because we want to win things, and at the start of the season we wanted to be a challenge for the top four.
“Many people were leaning against us, but quietly I had always hoped.”
Tottenham sinking Lester
Earlier, Tottenham Hotspur dealt a major blow to Leicester City’s aspirations in the Champions League as two goals from Harry Kane helped Jose Mourinho win 3-0 at home.
A special goal for James Justin from a strike by Sun Hyung Min gave Tottenham an early lead before England striker Kane scored twice twice before the break with superb goals after counter attacks.
There was no further scoring in the second half, leaving Leicester still in fourth place, but at the level of points with Manchester United, who has a match in hand before the teams face each other in the final of the season.
Third-placed Chelsea will also have a double reason to celebrate as pressure is eased after a surprise defeat against Sheffield United leaving doubts about qualifying in the top four.
Tottenham, who has improved a lot with one defeat since restart, moved to sixth in the challenge for the Europa League.
On the other side of the table, Bournemouth’s hopes of survival were influenced by a 2-0 defeat at home to Southampton, the southern coast neighbor.
Cherry was left facing a drop, three points behind Watford in 17, which made the shock decision to dismiss manager Nigel Pearson on Sunday, and confirmed the news in a statement to the club, with Hayden Mullins assuming a temporary coach in the last two games.