The Red Devils suffered a 3-2 defeat against Tottenham on Saturday and their performance levels have come under scrutiny in recent weeks.
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"The Champions League is completely different to the Premier League," Rooney told reporters. "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.
"I still think it is exciting. It is a great competition to be in. You look at the teams involved and it is a great challenge for everyone. It is something I love playing in."
United manager Sir Alex Ferguson has experimented against some of the continent's lesser-known sides in recent matches but assured fans that the club's established stars would play in Romania.
"We will play the experienced players on Tuesday," he said. "Cluj knocked out Basel over two legs in the qualifier and that shows where they are.
"They won in Braga, it's not an easy place to go to. We are taking it seriously. They think they can qualify now. They have a home game against United, get something and they are in a good position."