Clement Lenglet scored in the final minute of the game to give Barcelona a 1-0 win over Cultural Leonesa in the Copa del Rey on Wednesday.
A much-changed Barça side struggled to break down their third tier opponents but finally got their goal in the 90th minute when Lenglet headed home Ousmane Dembele's free kick.
Ernesto Valverde picked a completely different side from the one which hammered Real Madrid at the weekend.
Barça B trio Juan Miranda, Chumi and Jorge Cuenca started in defence, while there were also starts for the likes of Denis Suarez and Malcom.
However, Barça struggled to impose themselves in the final third and it was the home side who came closest to breaking the deadlock in the first half, Jasper Cillessen saving from Zelu.
The first half was marked by another injury to Sergi Samper. The midfielder was forced off with a calf injury, replaced by Carles Alena.
The Catalans didn't have their first shot on target until the 71st minute, when Alena tried his luck from distance.
Cillessen then made another good save, this time from Saul Gonzalez, before Barça turned the screw in the final few minutes.
Munir El Haddadi wasted a good opening, shooting over from 12 yards, before substitute Lenglet got on the end of Dembele's cross.
The two sides will meet again at Camp Nou in the second leg, with a place in the last 16 up for grabs.