Luke Shaw's own goal gave Barcelona a head start at the Old Trafford, thanks to a key VAR decision.
Luis Suarez was initially ruled out for offside after directing Lionel Messi's cross over Shaw and beyond goalkeeper David de Gea.
But after consulting VAR, Barcelona got a valuable away goal at the Old Trafford.
The hearts of the boat were in the mouth after Messi had fallen under a challenge from Chris Smalling – a bloody nose and an injured eye saw the Argentine go to therapy before returning.
Will Ole Gunnar Solskjaer's team transform this quarter-final of the Champions League into the front row?
Follow all the actions, as it happens, with our live updates below …
Man Utd XI: De Gea; Young, Smalling, Lindelof, Shaw; McTominay, Fred, Pogba; Dalot, Lukaku, Rashford.
Boat XI: Ter Stegen; Semedo, Piqué, Lenglet, Alba; Rakitic, Busquets, Arhur, Coutinho; Messi, Suarez.
45 + 2 minutesminute by minute
HALF-TIME: MAN UNITED 0-1 BARCELONA
45 minutesminute by minute
Visitors enjoy sustained pressure on the edge of the opposition box. Coutinho does well to put the ball in for Suarez after Messi, who was trying to make a brace, slipped. Suarez's shot ends up off target.
Get away a little towards the end of this half of Barcelona.
Valverde will ask for more in the second period.
Overall ownership ratio
43 minutesminute by minute
Barcelona have occasionally threatened in this first half, and forced De Gea to a good save, but United was the best team from Shaw's goal. There has been a lot of polls from the hosts without a real end product so far.
41 minutesminute by minute
I had no idea what Dalot was thinking there. He had to test Ter Stegen for at least five or six yards! Barcelona could do with the whistle at half time.
40 minutesminute by minute
GREAT CHANCE! United's pressure almost says as Rashford's cross is reached by Dalot in the six-yard box. Dalot has a large part of the goal to aim with the header, but instead tries to find a teammate by going over the door.
38 minutesminute by minute
Busquets is a lucky guy, overcoming another yellow postcard challenge on Pogba. United's French midfielder needs to continue doing what he's doing by taking a busquets tossed at every turn. The momentum is with United.
Another foul by Busquets.
A very thin line is walking.
36 minutesminute by minute
GREAT SAVINGS! Statistics show that Barcelona had more than 73% of the ball, but this is their first real attack from opening the scoring. Smalling can't manage a bouncing ball and Suarez queues up. After holding it in his hand, he sells Coutinho, whose powerful shot is kept out of De Gea's leg.
He's back now.
And it's getting ready again to make an impact.
34 minutesminute by minute
Dalot is having fun on the left against Semedo. The ball is worked for Pogba but his attempt to pass chipped is cut off. The United States has complete control of the game, although they still have to put Ter Stegen to the test with 34 minutes of play.
32 minutesminute by minute
Messi was given the green light to return to the field. Barcelona continues to look incredibly sloppy on the ball – can the Kingdom punish it?
31 minutesminute by minute
There was no malice on the part of Smalling when he captured Messi, but he left the superstar in a bad position. C & # 39; s blood dripping from the nose of Messi and will require some patches before returning to the field.
Put yourself down
VAR Songs at Old Trafford.
Scott McTominay went into the area, he used his feet well, but Gerard Pique touched the ball.
Meanwhile Messi is out of treatment.
He took a hit from Chris Smalling.
30 minutesminute by minute
PENALTY SHOUT! McTominay cuts in Pique in the box and there is a little bit of contact from the current Manchester United defender. The referee starts the appeals and VAR will not be used to check the decision. The game is then stopped while Smalling stumbles through Messi, taking his face in his arm.
Overall ownership ratio
29 minutesminute by minute
Barcelona is once again sloppy in front of its stage. Lenglet is caught on the ball and Young receives a cross in the box, which deviates to Ter Stegen to pick it up. Visitors, despite their advantage, look far from their best.
28 minutesminute by minute
Dalot breaks into the space left free by Semedo. It is reproduced on Pogba, which almost centers Lukaku's head six yards from the door, an inch or two less and the Belgian striker would have taken the lead.
Sergio Busquets has been booked.
United needs to keep running down the throat now.
He has never been the most mobile, and now he can't afford to make mistakes.
24 minutesminute by minute
Lukaku holds the ball on the left, but takes the wrong choice by entering the box, managing to find only Ter Stegen. The United States does not seem threatening enough in the last third.
26 minutesminute by minute
Shaw's own goal was the eighth Manchester United player who scored in the Champions League, with six of them at Old Trafford. It was also the first goal that the Premier League team conceded in a Champions League home game since September 2015 against Wolfsburg (fourth minute).
Both Avram and Joel Glazer are present tonight.
So far, they have seen their part largely dominated by visitors.
23 minutesminute by minute
SHOT! Rashford uses the Dalot bait race to his advantage. The space opens up for the United striker through the center, but his shot from outside the box goes over the crossbar. This is much better from the point of view of the hosts.
21 minutesminute by minute
The United States has the opportunity to counterattack following a bad kick routine of their opponents. Young has the ball on the right, but goes for an early cross to Rashford, finishing the ball directly off the pitch. Terrible all-round game!
L & # 39; goal. the goal …
Messi crossed it.
Suarez guided him.
Shaw has diverted him.
Coutinho dived (but failed to get close!)
18 minutesminute by minute
Fred takes the 25-meter free kick, sending it directly to the Barcelona wall. Young's hasty shot from follow-up is far from the target.
19 minutesminute by minute
Shaw stretches twice to prevent Messi from moving away from him and a yellow card is shown that will exclude him from the return match.
15 minutesminute by minute
The United gameplan had to hit Barcelona on the bench, playing almost like the outside. It will be interesting to see if those tactics change now that they are left behind. Dalot tries to move away to the left but Pique maintains his position and wins the ball.