I don't think Roma would be able to qualify to the semifinals, because they are playing with a team stronger than them. Milan have been quite consistent recently and I believe that they also have the Europa cup finals in heart. It will be a shock to me, if Milan is kicked out by Roma in the quarter finals.
Roma is doing pretty good, ever since the arrival of De Rossi, but that does not mean that when they face a strong club like Milan, they would be able to be victorious over them. Roman's strength is limited. Since Di Rossi took over from Mourinho, Roma have not won a big club if I am not mistaken. Let's not underestimate Milan who is sitting on the second position, while Roma is 5th on the table in their domestic league. Assuming, it was a different league, I would not doubt Roma.
Under normal circumstances I would agree with you, but there are two important points that speak against this. First of all, Roma today is not the same as Roma two months ago. They have done a lot to improve their game and it turned out positively in regards to their results. They did tie a game today which they should have won, but still they are on an uptrend and fighting for UEFA for qualifiers. Secondly, last season nobody expected them to go all the way into the final and yet they did while eliminating strong teams along the way.