After Joao Felix had hit the crossbar early on, Barca – still without the injured Robert Lewandowski – looked to have been left frustrated by a string of fine saves by Athletic goalkeeper Unai Simon.

But moments after being sent on by Xavi for his first senior appearance, the 17-year-old Guiu raced down the left channel to dispatch a composed finish and close Barca to within a point of leaders Real Madrid ahead of El Clasico next weekend.

Ahead of kick-off in Montjuic, both clubs paid tribute to former England and Manchester United midfielder Sir Bobby Charlton, who died on Saturday aged 86.

Athletic made a bright start, as Inaki Williams tested Barca goalkeeper Marc-Andre Ter Stegen from the edge of the box.

Barca almost took an early lead in the 10th minute when Ferran Torres cut into the penalty area and picked out Joao Felix, who swept his shot onto the top of the crossbar.

Athletic keeper Simon saved from Fermin Lopez after a surging run from left-back Alexandro Balde before – at the other end – Germany stopper Ter Stegen tipped a rising shot from Inaki Williams over the crossbar.

The visitors were forced into a change in the 26th minute when left-back Yuri Berchiche went off injured and was replaced by Inigo Lekue.

Joao Felix then saw his claims for a penalty waved away before Barca again had Ter Stegen to thank for keeping the scores level.

Athletic, managed by former Barca boss Ernesto Valverde, broke down the left through some quick passing from the Williams brothers.

Inaki Williams then fired in a low, angled drive which was pushed away by Ter Stegen before Inigo Martinez, facing his former club after joining Barca in the summer, then made a saving block from the follow-up shot by Nico Williams.

Ter Stegen was forced into action again in first-half stoppage-time when he made smart save with his feet to deny Nico Williams, who had again cut in on the angle from the left.

Athletic threatened again early in the second half, and again it was the Williams brothers who linked up before Inaki Williams fired the ball into the side netting.

As the hour mark approached, Simon made a fine double save to first deny Joao Felix and then Lopez from the follow-up after good work from former Manchester City midfielder Ilkay Gundogan.

Barca continued to press, but were again frustrated by another great stop by Simon, this time getting down to push behind a shot from Joao Cancelo, who had cut in from the right.

The breakthrough finally came with 10 minutes to go when substitute Guiu, who had only just been sent on for a LaLiga debut, raced onto a pass from Joao Felix down the left channel.

The 17-year-old took a touch in the penalty area before coolly dispatching his effort past Simon, who got an arm to the ball but could not keep it out.

Simon tipped a late header from Joao Felix over the crossbar as Barca closed out a hard-earned win.