The Portugal international had managed only 57 minutes of football for Atletico this season before Tuesday's last-32 first leg, but provided the telling moment of quality in Barcelona, while the likes of Antoine Griezmann, Diego Costa, Diego Godin and Koke were all rested.

Sant Andreu, who play in Spain's fourth tier, provided a solid test for Diego Simeone's side and should have snatched an equaliser through centre-back Joan Noguera late on.

Atletico itself squandered a handful of chances to make the win safe, but they will certainly be expected to complete the job at the Wanda Metropolitano on 6 December.

Gelson had tested goalkeeper Jose Segovia in a surprisingly even first half-hour, and it was the winger who gave Atletico the lead 33 minutes in, slotting home after just beating the offside trap to reach Vitolo's prodded pass.

Atletico suddenly looked in control and should have been given the chance to make it 2-0 from the penalty spot, but referee Jesus Gil Manzano was unmoved after Santiago Arias was scythed down in the box.

A rather pedestrian second half was dictated by Atletico, but it failed to make the most of its opportunities, with Nikola Kalinic seeing a shot blocked and Arias dragging an effort narrowly wide of the left-hand post.

Antonio Adan was forced to claw away a bouncing effort from Elhadji Bandeh in the closing stages, but it was Noguera who wasted Sant Andreu's best chance, heading the ball off the turf and against the crossbar with the goal gaping after he found space from a corner.