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.
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.
Maxi Gomez's smart finish gave them the lead before Iago Aspas made it two with a late penalty.
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 virtually 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, although a highly unlikely four-way tie involving Huesca and Real Valladolid could, theoretically, still save them.