The Gunners defeated Napoli 2-0 in last night's first-leg at the Emirates.
But Cech told RMC: "We respect the quality of Napoli, it's a fit team.
"In the first half, we made a perfect match, then we had to defend and be united. We could have scored more goals, but at 2-0, we take such a result.
"We are happy with our performance but we know that nothing is decided, it's not over yet."