Maybe their performance wasn't amazing there but that win is very important for the second leg anyway. Achieving that tough win without Lewandowski also deserves appreciation. Now they will need to focus on their next La Liga game which is against Valencia in Camp Nou. I think the players have learned their lesson from the last league game. We should be seeing improved performances this time. But on the other hand Lewandowski, Pedri and Gavi won't be able to play. However I still believe in them to win after seeing them beating Real Madrid away.
by the way, this pattern works for many teams - first present points to an outsider, then rob a favorite (or vice versa, first rob a favorite and then donate). It's good that this happened in the cup, and not in La Liga, otherwise the championship race is already over. At the moment, this victory for Barcelona has not affected anything.
Still, I don't see Barcelona to be that team that revolves there game around R. Lewandowski and so, his absence is undefined in the game.
Real Madrid on the other hand should have made this count for them at the very least. Barcelona remains a rivalry be it in what ever tournament, its an El-classico and they should punish them to have been top of the league already. Not the league for sure but, its an El-classico.