La Dea jumped to a 4-0 lead with Hans Hateboer's brace, Josip Ilicic and Remo Freuler individual efforts. A Denis Cheryshev away goal leaves some faint hope for Los Che ahead of the round 16 second-leg.
“We are on the one hand very satisfied, as we'd have happily accepted a scoreline like that," Gasperini told Sky Sport Italia.
“Yet we end up feeling regret over conceding that away goal, as we really shouldn't have made that mistake when 4-0 up and it wasn't the only situation like that.
“We need to be more precise when we go to the Mestalla. If we want to get into the Champions League quarter-finals, we have to play like this at the Mestalla too, prove we deserve to be there with a show of strength and win there too.
“We are a team who create many chances, it could perhaps have had more goals for us and them tonight. We created our goals, they sort of received gifts from us on theirs. We got a bit euphoric and lost sight of our concentration at times.
“We'll analyse now what we got wrong and learn from these errors, as we want to prove in the second leg that we deserve to be there.
“We've got to score there too. We will do some defending, but I don't want us to go to Spain and just sit on a three-goal lead, suffering the entire time."