Valladolid meet Barca this week after the Catalans were held 2-2 by Valencia on the weekend.
Sergio told Sport: "I want to say hello again. I met him during a preseason with Espanyol when we played a friendly against Southampton. He treated us very well, showing us the facilities and explaining many aspects of his club. It was not a formal greeting between two coaches, it was close and very friendly.
"He maintains his essence and his criteria above all else. He is being faithful to his style, to his scheme and that says a lot about a coach.
"I don't know (if Koeman is being brave) ... but he is being true to his principles."
On what Barca can expect from his Valladolid, Sergio added: "Compete, compete and compete ... I want us to be daring, to play football to force them to run back towards their goal."