They've conceded 8 goals in 9 games... and 3 of those were against Arsenal. Their defence has actually been their strength this season so far. Personally I think Bony will have to play incredibly well to get the best of Smalling who tends to play well against the stronger forwards but struggles against quicker, tricky strikers (like Aguero and Walcott). It's why I'd stick Sterling behind Bony and look for Bony to drag Smalling out of position (and Smalling is easily drawn out of defence) and then play Sterling in who will profit from the space in behind.
The result against Sevilla was good the performance not so much and that's a good thing. It means we'll be in good spirits but not overly confident as we know we'll have to play better. That, couple with a trip back from Moscow makes us favourites to win in my opinion. It will be interesting to see whether Rooney starts on Sunday.