Benfica were outstanding for the 4-0 win on Wednesday night.
Oblak later said: "Games like this can happen, but we can't afford them because we have to give a good impression. We've said many times that we started badly and today again. In the second half we didn't react and in the end these things happen. You have to start well from the start because if not the rivals are strong and they punish you.
"We didn't compete in the match, that's not the way it should be. We can't afford to do this and making excuses is pointless. We have to accept it and get to work because a difficult match awaits us on Sunday and if we're not at our best we won't get any points in San Sebastian (against Real Sociedad).
"I didn't have a single thing that I could save, we haven't been there and there's not much more that can be said."