Oscar came off the bench to score the winner eight minutes from time and leave last season's European champions well placed to reach the last 16, ahead of the second leg at Stamford Bridge next week.
Benitez said: "Eighteen attempts away is not bad but we have to be more clinical in attack.
"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.
"We have to create more opportunities and the strikers will score more goals.
"Fernando's (Torres) work rate today was really good and thanks to him the rest of the team, especially the second line of offensive players, could create.
"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."