Marcelino started his Athletic Club tenure with a 3-2 home defeat to Barcelona as the Catalans came from behind to lift themselves up to third place in La Liga.
It took Athletic Club just three minutes to open the scoring, with Inaki Williams played in behind the Barcelona defence by Raul Garcia to calmly finish underneath Marc-Andre Ter Stegen, scoring the first goal of the Marcelino era.
Their lead lasted just 11 minutes though as Frenkie de Jong hooked a deep cross back into the middle from the byline for Pedri to convert with his head with Unai Simon and the Athletic Club defence ball-watching.
And Barcelona completed the turnaround after 38 minutes when Lionel Messi and Pedri combined brilliantly, with the former passing into the back of the net as Simon was caught out of position.
A third goal was added after the hour-mark, with Messi crashing a finish in off the underside of the bar after being set up for the shot inside the box by Antoine Griezmann following good build up play by Pedri.
A late Iker Muniain goal gave Athletic Club a lifeline, but Barca held on for the win which lifts them above Real Sociedad and Villarreal in the La Liga table and to within five points of Real Madrid and seven of pace-setters Atletico Madrid.