The Gunners head to the Mestalla with 3-1 aggregate lead after the first leg.
But Cech doesn't believe that the scoreline gives his team a "decisive advantage" over Los Che.
"Over the course of this campaign we were in this position a few times," Cech said. "We lost the first round away at BATE Borisov so the home game could have been the last game in Europe.
"Then we lost away in France so the return leg against Rennes was the same thing.
"With Napoli, we won at home but we respected the opponent going to Italy. We knew it was going to be difficult and it could potentially be the last game in Europe for me.
"Tomorrow is exactly the same situation. We have given ourselves a two-goal cushion… it is an advantage but it is not a decisive advantage."