The United manager said: "I think there's a big motivation on Roma's part - I have seen some of the Italian papers and they are all about revenge.
"I can see it's a big occasion for them after the defeat they suffered last year. They will want to turn that scoreline around.
"But it was a one-off - it won't happen again. European football is just not that way nowadays, and it will be a very difficult game for us, Roma are playing very well at the moment."
"If I was them as soon as I had seen draw I'd have been delighted to have the opportunity to try to make it right so soon after," he said. "I would be sitting here rubbing my hands.
"I'm sure they will be fired up more than ever after last season's result so we will have to be aware of that."