Roma must overcome a 2-0 deficit, but Spalletti says: "We have to play for an hour as if nothing had happened at the Olimpico, completely ignoring the aggregate scoreline.
"We must do well to make the most of the chances we create. To lay siege to United would be counterproductive.
"After losing the first leg 2-0 it would be arrogant of me to come here and make proclamations. One must be humble and remember United are very strong.
"Looking over the past few years, there is a marked difference in the statistics and what has been achieved between these two clubs. Manchester are an example to be followed, but not just for Roma.
"If you don't fight against them, then you lose. In fact, you can fight and lose anyway, because they have quality as well.
"Nothing is impossible in sport. We have to give it our all, more than ever before. But I always start out thinking my team can overturn any result.
"We are playing this game and going out there in order to qualify. We certainly don't have problems scoring goals."