Goals from Ikechukwu Uche, Bruno Soriano and Denis Cheryshev saw the Spanish club triumph with a degree of comfort at the Liberty Stadium.
Speaking after the game, Monk stressed his side were trialling some new tactics but still lamented the mistakes made by Swansea.
"It was disappointing in terms of the result, we didn't play how we wanted to but during pre-season we have been working on a number of different things, so it was never going to be a completely fluid performance,” Monk told Swansea's official website.
"We made too many mistakes and the decision-making was poor."
Monk is confident Swansea will deliver a much better showing when they begin the Premier League campaign at Manchester United next weekend.
"It's been a very good pre-season," he said. "We've had some good performances and good training sessions and I think the fans will see a very good Swansea side this season.
"I'll do everything I can to make sure the players are prepared for next week and we'll go there [Old Trafford] full of confidence and firing on all cylinders.
"All I care about is our performances and the one thing I can say is that it will be better than today."
