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.
AS Roma current performance quit well under Daniele De Rossi in domestic league and Europe League, I have the same optimistic with you how potential AS Roma would be qualify to semifinal round for defeating AC Milan on quarter final match. Since arrived Daniele De Rossi, AS Roma tactical change drastically from more defensive become team more offensive and aggressive for scoring as many possibility goals every match.
Current Milan's performance still not consistency yet I think not problem for Daniele De Rossi continue the winning trend in Europe League, last match Roma failed taking three points after holding draw by Lecce and its bad thing get race fourth position standings and Roma have motivate for qualifying to Champion League next season.