You might be underrating Roma a bit. Their backline of Florenzi-Rudiger-Manolas-Emerson is, in my opinion, better than United's backline. In midfield they have Nainggolan, Paredes, and Strootman. All these guys are really good. In fact, any of them would solve United's three man midfield problem. United have the better attack on paper but Roma have been scoring goals for fun this season. I wouldn't be confident to back either team if they went up against one another, IMO.