Juan Mata gave the home side a deserved lead after 11 minutes only for De Gea to fumble Antonio Rudiger's 35-yard effort at Marcos Alonso's feet for the Chelsea defender to score an equaliser on the stroke of half-time. It meant United failed to take advantage of Arsenal's defeat at Leicester in the race for Champions League football with the point keeping Ole Gunnar Solskjaer's team sixth in the table -- three points behind Chelsea in fourth -- with just two games remaining.
Mata's goal was United's first from open play for nearly nine hours and would have won the game had it not been for another De Gea mistake, which led to a 13th consecutive match without a clean sheet -- their longest streak since 1970.
Mata has been in and out of the team all season -- under both Jose Mourinho and Solskjaer -- but he justified his selection against Chelsea. Playing at the top of a midfield diamond in the hole behind Marcus Rashford and Romelu Lukaku, he scored his first league goal since December. He was brave with the ball and worked hard to close down Jorginho. It was a timely reminder of what he can do as his contract standoff rumbles on.
De Gea is trying Solskjaer's faith. The United manager was adamant in his pre-match news conference that he has not considered dropping his goalkeeper, but De Gea has made mistakes in his past five games and in seven of his past nine United appearances. The one here -- spilling Rudiger's speculative effort at the feet of Alonso -- was one of the worst.
Manager rating out of 10
7 -- Solskjaer's rotation continued as he made five changes to the team that was beaten by Manchester City, although his hand was forced by injuries to Chris Smalling and Jesse Lingard. Got the better of Maurizio Sarri tactically, but his afternoon was spoiled by De Gea's error.
Player ratings (1-10; 10 = best. Players introduced after 70 minutes get no rating)
GK David De Gea, 4 -- The Spaniard made another horrendous mistake that gave Alonso the chance to equalise just before half-time, but made a late save from Gonzalo Higuain.
DF Ashley Young, 6 -- His delivery from corners caused Chelsea problems, but he could have provided more width on the right with Solskjaer employing a narrow midfield.
DF Luke Shaw, 7 -- Fantastic 50-yard pass into Lukaku in the opening five minutes before setting up Mata's goal with a energetic run from left-back and a clever pass from the byline.
DF Eric Bailly, 6 -- Started a game for the first time since he was hauled off after 36 minutes in Paris in March. Almost scored with a header from an Ashley Young corner before going off injured.
DF Victor Lindelof, 7 -- He didn't have much to worry about with Higuain so static, but he defended well -- as he has done for much of the season.
MF Nemanja Matic, 6 -- Criticised for his performance at Everton and dropped against City. Could be in trouble with the FA after a stray forearm caught Cesar Azpilicueta in the face in the first half.
MF Ander Herrera, 7 -- Back in the team after missing six games through injury. United have missed his energy and bite in midfield.
MF Paul Pogba, 6 -- Got stuck into the battle in midfield but never found the space to create anything meaningful.
MF Juan Mata, 8 -- United haven't lost a league game he has started since August, and he did his bit to continue the run with his first goal since January.
FW Marcus Rashford, 5 -- Tried to battle on after suffering a shoulder injury in the first half but was eventually forced off early in the second half.
FW Romelu Lukaku, 7 -- Lovely clip over the Chelsea defence to find Luke Shaw in the buildup to Mata's opener and caused plenty of problems with his strength and movement.
FW Alexis Sanchez, 5 -- On the fringes again.
DF Marcos Rojo, N/R -- Was lucky not to see red, and unlucky not to have scored.
MF Scott McTominay, N/R -- Saw nine minutes in replacing Mata.