Manchester United were denied victory against Barcelona thanks to Raphinha's equaliser at Camp Nou as Erik ten Hag's depleted squad impressed.
Marcos Alonso opened the scoring early in the second half, heading home a corner at the back post after sloppy marking from the away defence. The former Chelsea defender paid tribute to his late father in his celebration, after Marcos Alonso Pena died last week at the age of 63.
It was all square almost instantly, though, with in-form Marcus Rashford firing a low effort which beat Marc-Andre ter Stegen for pace. Rashford was involved again within minutes, breaking free down the right, and his low cross eventually found the back of the net via Barca's Jules Kounde.
It looked as though it could be the winner, only for Raphinha's cross from the right to evade everyone and find the back of the net. With away goals no longer in play in Europe, this tie is delicately poised going into the second leg.
After finishing second behind Real Sociedad in their group, Erik ten Hag's side were handed a tough task in their fight for a place in the last 16 of the competition. Barcelona went into the game sitting top of La Liga, with just seven goals conceded in 21 games, but finished third behind Bayern Munich and Inter Milan in the Champions League group stages.
Ten Hag's side went into the game missing a number of first-team stars, with Lisandro Martinez and Marcel Sabitzer suspended and several others injured. Barca, meanwhile, were without Sergio Busquets and Ousmane Dembele for the clash.
The hosts had the better of the early chances, with Robert Lewandowski testing David de Gea and Pedri sending a volley over the bar. United grew into the game, though, with Wout Weghorst and Marcus Rashford both bringing saves from Marc-Andre ter Stegen.
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Just as it looked like United had the upper hand, they almost gifted Barca a goal when Aaron Wan-Bissaka switched off, only for the right-back to deny Jordi Alba with a much-needed recovery tackle.
After the break, though, the game caught fire. Less than 15 minutes passed between Alonso's opener and United opening a 2-1 lead, with chances coming thick and fast.
Eventually the spoils were shared thanks to ex-Leeds star Raphinha, as Lewandowski failed to connect with his cross but it didn't matter. Here are Mirror Football 's talking points from a thriller at Camp Nou.
1. Rashford still unstoppable
It's not just Premier League sides who have found Marcus Rashford impossible to deal with since the World Cup. The England forward has been in scintillating form, and Barcelona found this out to their peril.
There had been glimpses in the first half, but Barcelona's opener appeared to spur him into action. First there was the equaliser, drilled like a rocket at Ter Stegen's near post, and then there was the dart down the byline to create United's second.
Barcelona didn't know what to do against the forward, who caused problems for both Kounde and Alonso at the back. The one blot on his copybook was a wild effort from the right of the box at 2-2, when many would have backed him to find the target.
2. Alonso goal shows football's fine margins
Early in the second half, it looked like we would see fine margins emphasised as Raphinha drilled a shot inches wide and then saw United on the attack moments later. We were right, but not for the reason we thought.
Jadon Sancho missed that opportunity, and roughly 60 seconds later the ball was in the other net. It could have been a cue for United to lick their wounds, but they weren't perturbed and instead found a response with their very next attack.
Previous incarnations of Man Utd might have been deflated by the opener. Not this version, though, and there will be hope that this kind of reaction can see them end their wait for silverware this season.
3. Shaw's centre-back test
With Martinez banned after picking up three yellow cards in the group stages, Ten Hag opted for Shaw over Harry Maguire or Victor Lindelof as his second centre-back. The former Southampton man has been capable in the role in the league, but repeating the feat in the face of Robert Lewandowski was a different matter.
Shaw was almost caught out inside the opening 10, allowing the Poland striker to get around his blind side before bringing a good save from David de Gea. Considering his long and storied career at the highest level, it may have come as a surprise to learn this was Lewandowski's first ever game against Manchester United, and the former Bayern Munich man tried to make up for lost time.
The one concern will be whether Shaw's impact at left-back has been missed. With Martinez back for the second leg, that could be one problem solved.
4. Ten Hag's formation shift
When the line-ups were released, United's front line looked relatively self-explanatory, but it proved to be anything but. Wout Weghorst started in a withdrawn role behind a central Marcus Rashford, with Jadon Sancho - rather than his England team-mate - on the left.
The manager experimented with the shift in the second half of the weekend win at Leeds, and presumably liked what he saw enough to use Rashford through the middle from the start this time. It very nearly paid off, too, with Weghorst breaking free from his deeper role but failing to find a way past Ter Stegen.
There was that wonderful chance for Sancho, too, just moments into the second half. The winger perhaps had more time than he thought from his outpost on the left, flashing a shot wide, and one wonders whether the outcome would have been different with Rashford pulling the trigger.
5. De Jong's task made tougher
For much of the summer, Frenkie de Jong looked as though he could start the season as a Manchester United player. The Dutch international was part of Ten Hag's fantastic 2018-19 Ajax squad which came within a minute of the Champions League final, but ultimately he opted to stay at Camp Nou.
There was almost a nightmare moment for the midfielder in the first half as he deflected a Jadon Sancho effort dangerously close to his own net. His task then grew, with Pedri forced off injured to join Busquets on the sidelines.
Gavi will also be absent for the second leg, with a yellow card earning the youngster a suspension. If Pedri also fails to recover in time, not to mention the already-absent Busquets, the Catalan club's options will be even more limited.
6. Casemiro and Varane show Clasico experience
No one needs reminding of the fierce rivalry between Real Madrid and Barcelona, and the boos which rang out at the mention of former Madrid stars Casemiro and Raphael Varane. Least of all the duo themselves, who locked horns with Barca plenty of times during their LaLiga careers.
Casemiro would have loved to have opened the scoring when he glanced a Bruno Fernandes free-kick goalwards half an hour in, but his touch sent the ball straight down the throat of Ter Stegen. As for Varane, a yellow card shortly before the break made for a testing second period in the face of Lewandowski's constant threat.
The Brazil midfielder also went into the book late on, while also coming close to scoring an own goal as a miscue came back off his own post. Overall, though, he showed exactly the kind of experience required with United missing so many.
