Smalling is wary of the away goal threat at Old Trafford following the 0-0 draw in the first leg in Marseille.
“It’s a bit of a dangerous scoreline with the way the away goals rule works,” he told United Review.
“It makes things interesting for the neutrals, but a bit nerve-wracking for the players and the fans.
“You could see that Marseille wanted to keep a clean sheet and they were well set up in that sense. They’re a good side. It was an edgy game over there with neither side wanting to give the other too much encouragement.
“From our point of view we need to keep things tight at the back because if they score it’ll mean we need to score twice.
“You don’t want to risk conceding the away goal, but I think the manager will want us to go at them.
“Being at home is a massive advantage for us and we’re confident of going out there and getting the result.”