Koeman was looking for a statement from his players at the Estadio Anoeta and early goals from Sergino Dest and Antoine Griezmann put them into a 2-0 lead at the break.
However, there was to be no let up from the visitors after the break, as Dest grabbed only his second league goal of the season, before Lionel Messi superbly made it 4-0 on the night.
With Barcelona rampant in the final half hour, French star Ousmane Dembele fired home Ilaix Moriba’s pass to make it 5-0.
Sociedad substitute Ander Barrenetxea did pull a fine consolation back for the hosts before Messi grabbed his second goal of the game to hand Koeman a first six-goal haul of the season.
The bulk of Koeman’s squad will now head off on international duty before a home game against Real Valladolid on their return to action in April.
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