Marc-Andre ter Stegen

The champions – who also lost at Celta last season – fell three goals behind in a dreadful first half that saw a poor clearance from Ter Stegen contribute to the hosts' opening goal through Pione Sisto.

A header from Gerard Pique and a Neymar penalty gave Barca hope in the second half, but Pablo Hernandez's header – after a more significant error from the goalkeeper – proved decisive prior to Pique's second.

The Spanish champions were busy before the transfer window closed last week, bringing in several high-quality players to improve the squad's depth.

Barca have since stated their next intentions are to extend the contracts of the likes of Lionel Messi, Luis Suarez and Ivan Rakitic in the coming months.

And now Bartomeu has added Ter Stegen to the list, despite the 24-year-old Germany international still with three years left on his deal at Camp Nou.

"[Ter Stegen] is a fundamental piece in the future of Barcelona," Bartomeu told RAC1.