Mario Hermoso and Marcos Llorente struck the goals in the 2-0 win in San Sebastian.
Simeone said, "You already know me and I will be recurrent with what I think, but the only thing that matters to us is the next game. The work is very good, but we have to show it in the next game against Getafe. It's the only thing that's going to count, really.
"Today we were very good in defence to be able to recover balls against a team that knows how to look for chances. We were able to control it with a lot of order and deployment, and that is due to the effort of the team.
"In the first half we lacked clarity in attack, but in the second half it gave us more clarity and shone in the final metres against a very difficult opponent."
Simeone has now overseen 300 victories as Atletico coach.
He added: "I knew I was going to return when I left as a player. And I prepared myself for all of this. But I also know they can kick me out tomorrow. But you have to consolidate all this, and you don't need the word or the imagination, but the reality, and that is seen on the pitch."