United go into their second Champions League fixture coming off the back of a 3-2 defeat to Tottenham Hotspur at the weekend.
In the competition last season United failed to get past the group stages and have already struggled to get the better of Galatasary in their European opener.
For these reasons Rooney says that they can't afford to take Cluj lightly.
"The Champions League is completely different to the Premier League," said Rooney to the club's official website.
"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."