Earlier, Real Madrid’s 22-game winning streak was ended by a 2-1 defeat to Valencia meaning both giants of the Spanish game were beaten on the same day for the first time since April 30, 2011.
However, despite their lowly league position of 13th, Sociedad have shown themselves well capable of upsetting the odds having now defeated Barca, Real and champions Atletico Madrid at home under three different coaches.
And Moyes believes his challenge now is to draw that level of performance out of his players on a more consistent basis.
“It is the happiest night since I arrived in San Sebastian. We worked very hard. The players showed courage and defended incredibly,” said the Scot.
“This team has shown it is capable in the big games against the best teams in the league and now we must show the same capability to win the other games. At home, for example, we have let a lot of points slip away.
“We defended well tonight and that is what we have to do in all the games, but in other games when we don’t have to defend so much we also have to pass the ball and attack better.”
Barca manager Luis Enrique paid a heavy price for leaving both Lionel Messi and Neymar amongst nearly EU€300 million (Dh1.3 billion) of talent on the bench.
Both South American stars only arrived back in Spain on Friday after an extended Christmas break and Moyes defended his counterpart’s decision to only introduce them after the break.
“The best players can’t always play at their best level in every match. The biggest games for Barcelona are still to come and in that sense I think Luis Enrique made the right decision.
“I was hoping this would happen, that some of their most important players wouldn’t start and we were prepared for that.”
Enrique also refused to accept that he had made an error with his starting line-up as Barca had more than 70 per cent of possession and 17 efforts on goal, but failed to take their chances.
“To feel sorry afterwards doesn’t help anyone. I took the decision to avoid risks with players that made a long journey only a few days ago.
“I thought it was better not to risk it and we will never know if things would have been different if I had made a different decision.
“I can only remember the goal and then us dominating. There weren’t that many chances, but I think we deserved more. We didn’t have the luck or the clarity we needed in front of goal.”
A third league defeat of the season means Barca remain a point behind leaders Real Madrid having also played a game more than the European champions.
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