Astana came from behind to beat Manchester United 2-1 in Nur-Sultan as a dream outing for Ole Gunnar Solskjaer's youngsters turned sour.
With qualification from Europa League Group L already secured, Solskjaer took the opportunity to field an entirely changed XI from the 3-3 draw at Sheffield United featuring seven teenagers.
Jesse Lingard took the captain's armband and gave United an early lead but the contest turned against the Premier League club after a horror miss from Tahith Chong in the 54th minute.
Left-back Dmitriy Shomko levelled less than a minute later and Di'Shon Bernard, one of three full debutants, put decisively through his own goal in the 62nd minute – sealing a famous win for the already-eliminated Kazakh champions.
United's youngsters began brightly but it was two of the older heads who combined for the breakthrough after 10 minutes – Luke Shaw cutting in from the left to feed Lingard, whose strike from 20 yards took a slight deflection to nestle in the far corner.
Astana should have equalised in the 22nd minute when Abzal Beysebekov's clever diagonal pass caught Shaw napping and Dorin Rotariu shot wastefully wide.
United then picked their opponents apart 10 minutes into the second half, with Shaw and Angel Gomes exchanging passes before Chong chipped over with the goal at his mercy.
And Chong paid for his wastefulness instantly when Shomko fired an unerring finish across Lee Grant into the bottom-right corner from Roman Murtazayev's pass.
A quickfire turnaround was achieved in the space of seven minutes after Antonio Rukavina raided unchecked down the United left and the unfortunate Bernard diverted the cross past Grant.
Runar Mar Sigurjonsson fizzed a half-volley wide as Solskjaer's side were almost picked off on the counter-attack in the 73rd minute.
United found a second wind and Astana needed heroics from goalkeeper Nenad Eric to repel a Gomes free-kick and deny Mason Greenwood from close range.
What does it mean? Encouraging glimpse of the future gives Solskjaer a problem
Before a lack of experience caught up with them after the break, United's youthful starting XI acquitted themselves very well. The rate at which Solskjaer gives these players further first-team opportunities will be intriguing as he is faced with a balancing act.
Throwing too many into a misfiring side would be unhelpful, but the sight of Gomes cleverly and inventively buzzing around the final third was a reminder of the creativity those more experienced professionals at Old Trafford too frequently fail to provide.
Lingard back on the goal trail
Handed the honour of captaining his boyhood club, Lingard led by example and deserved his first-half goal. That it was only his second strike of 2019 and first since January does much to explain why he has dropped out of the England reckoning. For a player who should be entering his prime as he turns 27 next month, this was hopefully a step in the right direction.
All goes wrong after Chong miss
The imposing midfielder was one of the key elements in United's impressive start, driving the visitors onto the front foot. But Chong's failure to make it 2-0 was a wretched and a highly costly error he is likely to stew on for some time.
Key Opta Facts:
- With an average age of 22 years and 26 days old, Manchester United's starting XI for this game was the youngest they have ever named for a match in major European competition (excluding qualifiers).
- Jesse Lingard scored just his second goal of 2019 for United, and his first in 29 appearances in all competitions since netting against Arsenal in the FA Cup in January, 307 days ago.
- United have lost a European match in which they scored the first goal for the first time in 25 such games (W20 D4) since losing 1-2 at Midtjylland in the Europa League last 32 in February 2016.
- This result ended a run of 15 Europa League games without defeat for United (W10 D5), and was their first in the competition since going down 1-2 at Fenerbahçe in the 2016-17 group stage.
A more familiar United line-up will host Aston Villa on Sunday at the start of a crunch seven days that includes Jose Mourinho's return to Old Trafford with Tottenham before the Manchester derby at the Etihad Stadium. The conclusion of Astana's domestic campaign means they are next in action when they wrap up the Europa League group stage at Partizan Belgrade next month.