Match ends, Leicester City 2, Manchester United 2.
Jose Mourinho criticised Manchester United's "childish" errors as his team fell further behind leaders Manchester City after conceding a 94th-minute equaliser from Harry Maguire at 10-man Leicester.
The Portuguese felt his side missed "joke chances" as they fell 13 points behind City - the biggest points gap between first and second place at Christmas in English top-flight history.
It was the second time in a week that United have conceded a goal in injury time, losing 2-1 to Championship side Bristol City in the EFL Cup quarter-finals.
United had looked set to take all three points after Juan Mata's double had seen them recover from Jamie Vardy's opener, while Foxes substitute Daniel Amartey was sent off for the home side 16 minutes and nine seconds after coming on, following two bookings in four minutes.
Anthony Martial and Jesse Lingard were particularly guilty of missing golden opportunities in the second half to put the game beyond Leicester.
Maguire caught the United defence out as they failed to deal with Marc Albrighton's deep cross into the box, the England defender running past the injured Chris Smalling to side-foot the ball past David de Gea from close range.
"We were punished by our mistakes," Mourinho told Sky Sports. "Childish loss of possession, so easy. Childish in their box and childish in our box.
"Half an hour after our second it is a game to finish four, five, six-one. We didn't win because we missed incredible chances, joke chances."
Wasteful United's missed opportunity
This result was a huge blow to any hope United had of catching City, who had earlier beaten Bournemouth 4-0.
And former England striker Alan Shearer believes the title race is "as good as over".
Speaking on Match of the Day he said: "Manchester City have lost twice in 2017 in the Premier League, they will not lose five games in the second half of the season."
Mourinho made eight changes from the side that lost to Bristol City, with Paul Pogba returning from suspension to captain the team, while Victor Lindelof, making his fifth Premier League start, went to right-back in place of the injured Antonio Valencia.
The Swede and centre-back Phil Jones were badly exposed by Leicester's counter-attack for the Foxes' opener.
Wilfred Ndidi sprayed a 60-yard pass to Riyad Mahrez, who only had Smalling and De Gea for company.
The Algerian held up the ball before sliding a clever pass to the arriving Vardy, who placed a low finish into the bottom corner for his 50th Premier League goal.
Mata equalised on 40 minutes with a precise finish, rolling the ball into the far corner from the middle of the box.
Lindelof blocked Christian Fuchs' goalbound effort after the break, while Martial missed a one-on-one with Kasper Schmeichel, blasting over from 12 yards.
Spanish midfielder Mata got his second on the hour mark, beating Schmeichel with curling a free-kick after Martial had been fouled on the edge of the box.
United dominated in the second half and the result should never have been in question with 19 attempts on goal in the match.
Lingard should have made it three when he hit the post after rounding Schmeichel, while the Dane also got his fingertips to the ball to stop substitute Marcus Rashford when he was through on goal.
United failed to capitalise on Amartey's 73rd-minute sending off and Mourinho looked to shore up the lead instead of extending it, bringing on Ander Herrera for Lingard with 15 minutes left in the match.
"I don't get United," said ex-England winger Chris Waddle on BBC Radio 5 live. "They've had so many chances to really put this game beyond Leicester but they've messed about and missed chances."
'Good result for the future'
Leicester had also suffered EFL Cup defeat in midweek, losing on penalties to Manchester City.
Claude Puel made seven changes, with Mahrez and Vardy restored to the starting line-up.
The pair were crucial to the Foxes' gameplan and, after soaking up United's pressure, they counter-attacked at will.
After United took the lead they were fortunate not to fall further behind, but showed character to come back and equalise.
"It was difficult but with our positive attitude we stayed in the game and believed in ourselves," Puel told Match of the Day.
"It's a good result for all of us because the belief keeps going. It's fantastic for the future.
"Now we know if that situation happens again we can score with 10 men against 11, so that's a good feeling for the future."
Their revival under Puel leaves them in eighth at Christmas, but with only two defeats in his 10 league games in charge.
Man of the match - Juan Mata
Stats - Vardy's milestone
- Vardy scored his 50th Premier League goal in this match - the first player to score 50 in the competition for Leicester City.
- 42% (21) of Jamie Vardy's 50 Premier League goals have come against 'big six opposition'.
- Since August 2014, only Sergio Aguero (21 in 30 appearances) has scored as many goals in the Premier League against fellow 'big six' opposition as Vardy (21 goals in 40 appearances).
- Mata has been involved in five goals in his last four Premier League appearances against Leicester City (four goals, one assist).
- Three of the nine Premier League goals scored from direct free-kicks this season have been netted against Kasper Schmeichel.
- Amartey's dismissal was the quickest a substitute has been sent off in the Premier League since Ben Watson for Watford at Southampton in August 2016 (12 minutes, eight seconds).
Manchester United host Burnley at 15:00 GMT on 26 December, while Leicester play at Watford at the same time.
- 2SimpsonSubstituted forAmarteyat 57'minutesBooked at 73mins
- 21IborraSubstituted forOkazakiat 67'minutes
- 26MahrezBooked at 81mins
- 7GraySubstituted forChilwellat 80'minutes
- 11AlbrightonBooked at 54mins
- 1de Gea
- 2LindelöfBooked at 34mins
- 31MaticBooked at 47mins
- 8MataSubstituted forMkhitaryanat 83'minutes
- 14LingardSubstituted forHerreraat 76'minutes
- 11MartialSubstituted forRashfordat 71'minutes
- Jonathan Moss
- Shots on Target
Second Half ends, Leicester City 2, Manchester United 2.
Goal! Leicester City 2, Manchester United 2. Harry Maguire (Leicester City) right footed shot from the right side of the six yard box to the bottom left corner. Assisted by Marc Albrighton with a cross.
Delay over. They are ready to continue.
Delay in match Chris Smalling (Manchester United) because of an injury.
Offside, Manchester United. Nemanja Matic tries a through ball, but Romelu Lukaku is caught offside.
Phil Jones (Manchester United) wins a free kick in the defensive half.
Foul by Wilfred Ndidi (Leicester City).
Attempt missed. Marc Albrighton (Leicester City) left footed shot from outside the box misses to the right following a corner.
Corner, Leicester City. Conceded by Ashley Young.
Foul by Henrikh Mkhitaryan (Manchester United).
Wes Morgan (Leicester City) wins a free kick in the defensive half.
Henrikh Mkhitaryan (Manchester United) wins a free kick on the right wing.
Foul by Christian Fuchs (Leicester City).
Phil Jones (Manchester United) wins a free kick in the defensive half.
Foul by Shinji Okazaki (Leicester City).
Substitution, Manchester United. Henrikh Mkhitaryan replaces Juan Mata.
Offside, Manchester United. Chris Smalling tries a through ball, but Juan Mata is caught offside.
Riyad Mahrez (Leicester City) is shown the yellow card.
Riyad Mahrez (Leicester City) has gone down, but that's a dive.