Rasmus Hojlund's record-breaking double was enough to earn Manchester United a tense 2-1 win at Luton - a fourth league victory in a row to take Erik ten Hag's side closer to the top four.
The 21-year-old became the youngest Premier League player to score in six straight games as he put United two up inside seven minutes - clinically finishing after 36 seconds following Amari'i Bell's mistake, before instinctively chesting in Alejandro Garnacho's shot from a corner.
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Carlton Morris rounded off a scintillating opening 15 minutes by pulling one back and Luton should have levelled with several big chances as they put United on the ropes, with Casemiro lucky not to see red for lunging on Ross Barkley while on a booking.
United were more settled after the break - with Casemiro taken off at the interval - and should have put the game to bed as Bruno Fernandes and Garnacho both failed to score despite both rounding Luton goalkeeper Thomas Kaminski.
The hosts pushed for an equaliser until the last kick of the game - with Barkley hitting the bar - but Rob Edwards queried the amount of added time in the second half, saying: "I don’t know where that one came from. Man Utd are winning 2-1 so it’s four minutes."
But United held on to move within three points of fifth-placed Tottenham, with a top-five spot possibly being enough for a Champions League place next season.
Asked about Hojlund's scintillating form in front of goal, with the striker now on eight goals in his last seven games in all competitions, Ten Hag told Sky Sports: "We recruited him as he's a strong character. Although he's 21 he can really perform under stress. That's what you need as a Manchester United striker.
"He doesn't get nervous or lose confidence when he's not scoring. Once you are in a run, you see it, now he's scoring and he's in. That's how he is now. He has a lot of confidence and I'm sure he'll score more."
A player on fire - Hojlund's record-breaking double!
How Man Utd edged it - again
It took just 36 seconds for Hojlund to make history. Bell's slack, square pass along the backline was picked up by the Danish forward. He had the simple task of skipping past Kaminski and slotting home.
The United onslaught continued, as Luton struggled early doors. Casemiro was denied with a block from a corner, which was cleared as far as Marcus Rashford whose shot was saved by Kaminski. Garnacho also stung the Luton goalkeeper's palms in a similar fashion.
But another shot from distance ended up bringing United's second. Garnacho's fierce effort was chested on by Hojlund in a wonderful improvised finish.
Then came the attempted Luton fightback as former United midfielder Tahith Chong saw a shot deflected into the path of Morris, who beat Andre Onana to the ball to head home.
All of a sudden, the game changed as it was Luton on the attack, with United now on the ropes. Morris and Gabriel Osho put free headers over the bar from set-pieces, before Cauley Woodrow saw a clear-cut chance in the box blocked after Harry Maguire cheaply gifted the ball to the impressive Barkley in his own half.
United came under more pressure as Casemiro, already booked for a challenge on Bell, avoided a second yellow for lunging at Barkley. Luke Shaw also picked up a booking and was forced off before half-time. Meanwhile, Alfie Doughty should have brought Luton level by firing wide from the edge of the box after cutting inside.
Ten Hag took Casemiro and Maguire off at half-time and United did seem more controlled after the break.
Diogo Dalot was picked out by Onana in the box and Kaminski smothered the effort. The Luton goalkeeper was equal to fierce strikes from Rashford and Garnacho either side of the hour mark.
United had two chances to seal it, however. The first fell to Bruno Fernandes who rounded Kaminski but was denied by a last-ditch block by the retreating Albert Sambi Lokonga. Garnacho did exactly the same on the break moments later but dallied as he cut back inside and Luton cleared with the goal gaping.
Fernandes put another big chance wide of Kaminski's far post as many began to question Luton would make them pay. They nearly did in the final minute of stoppage time as Barkley headed on a corner - only for it to crash off the bar and over, with Onana scrambling.
Another example of a valiant defeat for Luton.
Ten Hag: Bookings for Casemiro are crazy!
Ten Hag revealed after the game that he took Casemiro and Maguire off at half-time because he felt the sense that the referee would soon send them off for their next foul.
"I saw the reaction of the referees and I thought the next one [foul] they would go off. I didn't take the risk. We had good replacements in [Scott] McTominay and [Jonny] Evans. It was the right decision as the performance of those two was very good."
Later in his press conference, Ten Hag claimed Casemiro is unfairly treated as officials are booking him when he does not touch opposition players. Last week, the Brazilian was bemused for being booked for a foul on Aston Villa's Matty Cash with minimal to no contact on the player. On Sunday, he was cautioned for a foul on Luton defender Bell.
"Even when he is not touching opponents, he gets booked," Ten Hag said. "Often on the first moment in the game as well. It was crazy today.
"The first booking was so unfair. The second? It could have been given. That's why we took him off. But it's difficult to play when you are not even touching an opponent and get booked.
"Now he is back in the team, he gives so much to the team. So much calmness and organisation. We are very happy, because since he is back, the team has lifted its levels."
Edwards: Man Utd got lucky with red | Adebayo injury rocked us
Meanwhile, Luton manager Rob Edwards felt United "got away with one" regarding the Casemiro incident - but refused to blame the officials for his side's defeat.
"The fact he came off at HT probably tells you that," the Hatters boss said. "They [United] are saying first [yellow card] wasn't [one], but just because the first one wasn't that doesn't mean second one isn't. Maybe they got away with one there.
"We shot ourselves in the foot, so I won't blame the officiating."
On only four minutes of injury time being played in the second half: "It was back to last season, wasn’t it? I don’t know where that one came from. Man Utd are winning 2-1 so it’s four minutes.
"But I’ve got to give the players a lot of credit. They showed an air of calmness and belief, commitment to the game plan and dragged themselves to a really good performance. We got back into the game and pushed.
"There’s lots to like about the performance but you can’t give a team like Manchester United a two-goal headstart. We got punished with a couple of errors and it was hard to recover from that."
On losing top scorer Elijah Adebayo to an injury in the warm-up: "Losing your No 9 and then going 1-0 down after 40 seconds is not the ideal way to start against Man Utd.
"I think it rocked and wobbled us for a little bit. When 1-0 turns to 2-0 after six minutes, you're fearing the worst and you go 'how do we respond to it?'
Analysis: Changing of the guard? Mainoo excels at Casemiro's expense
Sky Sports' Sam Blitz at Kenilworth Road:
Manchester United put out two midfielders at Luton. One was 18 years old, the other a four-time Champions League winner. One of them was lucky to stay on the pitch by half-time, the other looked cool and composed.
Kobbie Mainoo's expert display showed Casemiro, a team-mate 13 years his senior, how it's done.
Mainoo was superb as United calmed down a frantic game at Luton after half-time and hung on for a win. The midfielder won nine possessions in the defensive and middle thirds fn the pitch, more than any of his United team-mates by a country mile. He also ran past an opposition midfielder six times out of the seven times he tried it.
Casemiro, meanwhile, failed to run past any of his opposite numbers or win a single ball in the United defensive third.
As Jamie Redknapp said after the game: "It was too much for Casemiro" but the "real deal" "Mainoo had the "youthfulness and the presence" to cope with it. Sunday's game at Luton could herald a changing of the guard in the United midfield.
What's next?
Luton are back in Premier League action on Wednesday as they travel to Liverpool; kick-off 7.30pm.
Man Utd play again on Saturday as they welcome Fulham to Old Trafford in the Premier League; kick-off 3pm.
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