I believe more in AC Milan than in AS Roma but I know AS Roma get good performances with De Rossi as their new coach. He is doing his job well with Roma but he has very little experience as a coach, less than other coaches in Leverkusen, Liverpool and AC Milan. It can be his weakness and can cost Roma in important matches.
Among four teams, Liverpool, Leverkusen, AC Milan and Roma, I would like to pick either Liverpool or AC Milan as best candidate to win Europa League. AS Roma and Leverkusen have good strength and can win Europa League but less than Liverpool and AC Milan. Leverkusen also will have to beat Qarabag in Germany to go to quarter finals and with a first match between two teams, Leverkusen will have to beat a tough opponent.
The Serie A teams have impressive performed in UEFA Europe League first leg 16 of round except the draw result from Atalanta but its not really bad with their away match. AS Roma and AC Milan are guarantee teams will qualify to next round or quarter final after bigger score winning in the first leg and make easily for second legs match indeed with away match. Just reminder for AC Milan have conceded two goals on the first leg and margin two goals capital on second legs match should improve their performance and not losses above three goals if won't eliminated in 16 round of Europe League.
For AS Roma seems guarantee qualify to next round, have capital four margin goals on the first leg without any conceded one goal yet seems not much problem on second leg match although got losses under three goals and I don't think Daniele De Rossi will playing more defensive.