Barcelona rested all 11 players that started against Liverpool in the Champions League but paid the price on Saturday as they were beaten 2-0 by struggling Celta Vigo in La Liga.
Lionel Messi, Luis Suarez and Philippe Coutinho were all among those left out ahead of the return at Anfield on Tuesday but Barca were not unscathed, after Ousmane Dembele hobbled off early in the first half.
The club later confirmed Dembele sustained a hamstring injury and will undergo tests on Sunday but it seems unlikely the Frenchman will be fit to face Liverpool in the semi-final second leg.
Dembele missed almost four months with a hamstring strain last season and suffered from problems in the same area in January and March, sitting out for another month each time.
Defeat will hardly trouble Barcelona, who wrapped up their 26th La Liga title last weekend, but victory comes as a major boost for Celta, who move five points clear of the bottom three with two games left to play.
Real Valladolid and Girona, both just beneath them, may feel disappointed by Ernesto Valverde’s team selection, particularly given his side took six and four points off them respectively this season.
The result also means Rayo Vallecano are relegated after they were thrashed 4-1 by Levante earlier on Saturday. Rayo needed Celta, the only team they could catch, to lose all three of their remaining matches.
Barcelona will now quickly turn their attention back to Liverpool, who at the same time on Saturday were playing Newcastle in the Premier League, making only four changes to their own line-up from the side that lost 3-0 at Camp Nou on Wednesday.
Liverpool won 3-2 but Mohamed Salah was stretchered off in the second half.
Barca’s changes created the strange spectacle of Celta giving a guard of honour to youngsters like Jean-Clair Todibo, Carles Alena, Moussa Wague and Riqui Puig before kick-off.
Dembele lasted only six minutes and was replaced by Alex Collado, the 20-year-old midfielder thrown on for his first-team debut.
Puig was perhaps the most impressive of the fledglings and he teed up Barca’s best chance in the first half but Wague’s shot was well saved by Ruben Blanco.
Celta came on strong in the second 45 and thought they had taken the lead when Gustavo Cabral’s header was palmed out by Jasper Cillessen to Nesto Araujo, who finished. Araujo was shown by VAR to have been a fraction offside.
But Celta did not have long to wait. In the 67th minute, with Todibo off injured, Barca were briefly down to 10 and Maxi capitalised, brilliantly volleying in Ryad Boudebouz’s cross in at the near post.
There was still time for one more in the 88th minute as VAR spotted a handball by Wague in the area and Aspas converted to confirm a crucial win.
Atletico Madrid had earlier missed the chance to ensure they finish above Real Madrid for a second consecutive year after they slipped to a surprise 3-0 defeat by Espanyol.
Atletico needed only a point to guarantee second place in La Liga, with two games left to play, but Espanyol were deserving winners at the RCDE Stadium thanks to Diego Godin’s own-goal and two strikes from Borja Iglesias, the second a penalty.
“The first goal changed the game, the second one made us worse,” said Atleti coach Diego Simeone.
“Espanyol were deserving winners. They were better and they had more chances,” he added.
Real Madrid face Villarreal on Sunday but a third-place finish seems inevitable.
To overtake their city rivals, Real will need to win all three of their remaining games and hope Atletico lose both of their last two.