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.
I also predict that Roma will not be able to get rid of Milan in the quarter finals of the Europa League, even though De Rossi has succeeded in polishing Roma to be better, the difference between the two squad depths and their consistency so far makes us more confident in Milan. If Pioli fails to bring Milan to the semi finals, I predict he will be replaced next season, he has also failed to bring Milan through to the Champions League group stage.
Milan has a good reputation when playing in European competitions, but this season the competition is very tight, because the other two teams, namely Liverpool and Leverkusen, appear more consistently than them. However, it does not rule out the possibility that Milan can advance to the final, even though we know that this effort will not be easy to do. If Milan fails to win the Europa League trophy, then they will end the season empty handed this season.