The Basque youngster takes Xisco Nadal's record...
At 16 years and 289 days old, Muniain has broken the record previously held by Xisco Nadal.
Muniain, who is nicknamed 'Bart Simpson' in the Athletic dressing room, was brought on as a substitute against Valladolid but soon made an impact when he capitalised on a loose ball in the hosts' penalty area in the 77th minute.
The youngster has already found the net twice in the Europa League for los Leones, but has now scored in the Primera at a younger age than the likes of Barcelona's Bojan Krkic and Real Madrid legend Raul.
Meanwhile, worries over Fernando Llorente's head injury have been calmed by the news that there are "no complications". Athletic's medical staff told Marca that the striker would now rest at home for two or three days.
Anthony Wright, Goal.com