The 27-year-old admits there were no offers for his services this summer.
“The Champions League is one of the factors in my decision to stay with Rangers,” he told the Daily Record.
“But I have to say I stayed put at the club because there was no serious offer to buy out my contract.
“A game against a team like Manchester United motivates any player. It will be the second time I’ve faced Rooney but he’s not the only threat. There are other strikers who are rated dangerous, such as Dimitar Berbatov.
“In a team like Man United the danger can come from any player and that’s why it’s so special to play them.
“It can also help us because even though our group is harder than last year’s – with United, Valencia and Bursaspor – we’re highly motivated and want to do better.
“Our aim is to qualify for the last 16 or, at the very least, finish third to go on and play in the Europa League.”
