Raul Rusescu converted a 34th-minute penalty after he was pulled back by Ryan Bertrand to give the Romanians a 1-0 advantage in the last-16 tie ahead of next week's second leg at Stamford Bridge.
Before then, last season's Champions League winners continue their FA Cup defence with Sunday's quarter-final at Old Trafford, a match Benitez insists his players were not distracted by while struggling in Bucharest.
"It's another competition and atmosphere," Benitez said.
"It will be totally different. It's another challenge for us."
Just like United, Chelsea warmed up for the FA Cup clash with a European defeat, but at least their rivals were defeated by Real Madrid.
Chelsea's priority is a top four finish in the Premier League and Champions League qualification, which may be just as well after the loss in Romania's capital.
Benitez made five changes from the side which beat West Brom, with John Terry included in central defence.
Given the captain's fitness travails, it is likely he will not be in the first XI at Old Trafford.
Terry may next be called upon for the return with Steaua, with Benitez confident of advancing to the quarter-finals.
"We know that we have to do our job now at home," Benitez said.
"In these kind of games and this kind of competition, if you don't score away it's always more difficult. Still you have to believe.
"We are disappointed, but still we have the belief we will win at home."
The decisive moment came when left-back Iasmin Latovlevici, earmarked as a dangerman by Benitez on the eve of the match, crossed with the outside of his right foot towards Rusescu, whose attempts to reach the ball were thwarted by a pull from Bertrand.
Referee Sergey Karasev booked the left-back, playing in place of Ashley Cole and exposed at the far post, and Rusescu sent the resulting spot-kick under Petr Cech.
"I think it was very soft," Benitez told ESPN.
"To give a penalty in this situation, and especially when you have so many referees in their positions, it is soft.
"You cannot change the situation but it is very soft.
"If you give the penalty, why not the (red) card? Strange."