Xavi's side slumped to a 4-1 loss to Real Madrid in the recent Supercopa final, and their hosts - the lowest-ranked team left in the competition - will aim to prolong the Blaugrana's blues at Renia Sofia.
Match preview
Fewer than 5,000 spectators will pack in to Unionistas' humble home ground for one of the hosts' most momentous matches in recent memory, which they earned with a spectacular penalty-shootout success over Villarreal in the Copa del Rey last 32.
After Alfred Planas cancelled out an opener from ex-Barcelona academy product Ilias Akhomach, Unionistas goalkeeper Ivan Martinez denied both Alberto Moreno and Kiko Femenia from the spot, while the third-tier minnows found the net seven times from 12 yards to complete a remarkable upset.
Gernika and Sporting Gijon have also bowed to the superiority of Dani Ponz's men in this season's Copa del Rey, where Unionistas - who were only established 10 years ago - will be competing in the last 16 for the very first time, on their fourth appearance in Spain's premier knockout competition.
Two second-round exits and a third-round elimination at the hands of Real Madrid preceded Unionistas' magical run in 2023-24, and since being baptised in the sixth tier of the Spanish footballing pyramid, the hosts have firmly established themselves as a third-tier side.
Ponz's troops prepared for Thursday's highly-anticipated battle with a 2-1 beating of Rayo Majadahonda in Saturday's Primera Division contest, where Hector Nespral and Ramiro Mayor did the damage, but little can prepare Unionistas for the visit of the Blaugrana behemoths.
More wood was added to the fire surrounding Xavi's future in the Barcelona hotseat following a failed Supercopa de Espana venture in the Saudi Arabian city of Riyadh, where La Blaugrana cruised past Osasuna in the semi-finals for the right to fight Real Madrid in El Clasico.
However, with just 10 minutes gone, Xavi's men were already two goals down - Vinicius Junior tapped home into an empty net on both occasions after the Blaugrana's high line was exposed - but Robert Lewandowski soon pulled a goal back with a thumping strike into the bottom corner.
The Pole's celebrations were premature though, as a calamitous second half saw Vinicius complete his treble and Rodrygo also get in on the act for Los Blancos, who played the final 20 minutes with a numerical advantage thanks to Ronald Araujo's dismissal for two bookable offences.
Returning to base with their tails tucked between their legs, Barcelona did at least come out on the correct end of a five-goal scoreline in their Copa del Rey last-32 tie with Barbastro - although their 3-2 success was far from exceptional - and their scoring streak stands at a respectable 13 matches as they bid to avoid a second last-16 exit in three seasons.
No team has won more than Barcelona's terrific 31 Copa del Rey crowns, but if the Blaugrana are to relieve some of the pressure on their manager, avoid a historic giant killing and further their chances of that number ticking over to 32, a performance akin to Sunday's cannot be repeated.
Unionistas Copa del Rey form:
W
W
D
Unionistas form (all competitions):
Barcelona Copa del Rey form:
W
Barcelona form (all competitions):
W
L
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W
W
L
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Team News
Unionistas right-back Sergio Camus was withdrawn at half time of his side's last-32 success over Villarreal, but that was seemingly a tactical alteration from Ponz, as he had been cautioned with just 11 minutes on the clock.
The hosting coach rewarded goalscorer Planas with a start against Rayo Majadahonda in league action, but he failed to kick on after netting against the former Europa League winners, being taken off with 65 minutes on the clock.
A recall for Juan Serrano is therefore not beyond the realm of possibility, while Nespral's midfield partner should be 20-year-old Javi Villar, on loan from Real Madrid's Castilla unit.
As for Barcelona, the naughty step will hold two players on Thursday; Araujo must serve a one-game ban for his sending-off against Real Madrid, while Lamine Yamal completes a two-game cup suspension relating to a previous misdemeanour at youth level.
In addition, injury victims Gavi (ACL), Marcos Alonso (back), Marc-Andre ter Stegen (back), Raphinha (hamstring) and Inigo Martinez (hamstring) remain on the treatment table, while Joao Cancelo (knee) is highly unlikely to make the cut too.
The manner of Barca's performance against Barbastro was far from convincing, but with so little time to prepare, Xavi should not be averse to deploying young talent such as Marc Casado and Vitor Roque at Renia Sofia.
Unionistas possible starting lineup:
Martinez; Camus, Mayor, Gimenez, Jimenez; Nespral, Villar; Planas, Serrano, Gomez; Slavy
Barcelona possible starting lineup:
Pena; Fort, Kounde, Christensen, Balde; Lopez, Romeu, Pedri; Torres, Roque, Felix
We say: Unionistas 1-3 Barcelona
The shackles will well and truly come off for a Unionistas side who ought to play without any trepidation on Thursday evening, especially with Barcelona's morale and energy levels being drained in Saudi Arabia.