Although United slumped to a lacklustre 3-2 defeat against Tottenham on Saturday, their first home loss to the north London club for 23 years, they are widely expected to get back on track against the unheralded Romanian champions on Tuesday.
But Rooney, who came on as second half substitute against Spurs, knows United can't afford to take any opponent for granted in the Champions League.
Last season, Sir Alex Ferguson's side crashed out at the group stage after underestimating a section that looked less than threatening on paper.
Rooney is determined to ensure United avoid a repeat of that embarrassing exit and he has demanded an much-improved and more focused display against Cluj in Transylvania.
"The Champions League is completely different to the Premier League," Rooney said.
"If you are not at your best you get punished, which is what happened to us last season.
"We have to make sure that won't happen again this year so this is a big game for us.
"Hopefully, we will get a win which will put us in a good position to go through so early on in the group."
United boss Ferguson has changed his team's European preparations this season, opting to fly out later for away games to allow more time on familiar home terrority.
That will give Ferguson a chance to settle on his starting line-up to face Cluj before departing and Rooney is likely to feature strongly in his plans after an impressive display in the Spurs defeat.
Rooney's enthusiasm for the big stage will be another factor in Ferguson's thinking and the 26-year-old admits he still relishes the chance to shine in the Champions League after so many years in the competition.
"It still think it is exciting. It is a great competition to be in," he said.
"You look at the teams involved and it is a great challenge for everyone. It is something I love playing in."
United announced later on Monday that Ferguson had opted to leave Ryan Giggs, Michael Carrick and Paul Scholes out of his squad in order to keep them fresh for this weekend's Premier League fixture against Newcastle.
Antonio Valencia, who has an ankle injury, was also absent from the squad, but Northern Ireland defender Jonny Evans was included after training on Monday as he shook off a dead leg suffered against Spurs.