The Blues take a one-goal lead into the second leg of the last-32 tie after Oscar came off the bench and found the net, but the Spaniard was not completely satisfied.
"Eighteen attempts away is not bad but we have to be more clinical in attack," Sky Sports News quotes him as saying. "The first 20 minutes we were in control. We had some half chances. We were in control.
"After this 20 minutes they were pushing and pushing. The team, as a team, had problems defending.
"We changed a little bit in the second half. I think the concentration of the team was much better, the intensity in defence was much better and then we had more control."
Benitez singled out under-fire striker Fernando Torres for praise, insisting that his work rate was impressive even though he failed to score.
"Fernando's work rate today was really good and thanks to him the rest of the team, especially the second line of offensive players, could create," he added.
"His job was push and run and keep their defenders deep so he could create space for the other players.
"Everybody expects from a striker just to score goals but he's doing also a great job for the team."
The return leg takes place at Stamford Bridge on February 21.