The Red Devils slipped to a shock 3-2 defeat at home to Tottenham on Saturday, conceding two goals in the first-half.
And the 70-year-old wants more concentration from his team as they travel to Romania for what could prove to be a tricky clash, following the hosts' impressive 2-0 away win at Braga in the opening round of the group stages.
"I hope the first-half on Saturday was [an issue of] awareness," he told reporters.
"When Jonny [Evans] and Rio [Ferdinand] played consistently every week last season [from December when Nemanja Vidic was injured], we got better.
"As part of our preparations we have to assess the opposition and we saw their performance in the last game against Braga.
"[Before that] they knocked out Basel, beating them twice. We know from our own experience last season that Basel knocked us out. That gives us every respect for Cluj because it tells you they’re a good team.
"Cluj defend quickly and get plenty of bodies behind the ball. Their main strength is counterattack, as they showed against Braga."United will be short of numbers in midfield for the trip to Romania with the experienced trio of Ryan Giggs, Paul Scholes and Michael Carrick all remaining in Manchester.
"Carrick didn't report today, he's in bed with a virus," Ferguson added.
"Ryan [Giggs] also felt rough this morning and we sent him home. We left Paul Scholes behind.
"Those three players are missing - two with a virus and Paul Scholes because, although he looked great this morning and played great on Saturday, at 37 years of age I can't expect him to play three games [in a week]."