The Red Devils failed to advance from the group stages of the competition last year, and after struggling past Galatasary in their European opener, Rooney says that they cannot afford to take Cluj lightly.
"The Champions League is completely different to the Premier League," said Rooney. "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."
Rooney came off the bench on Saturday after a month out through injury.