Honestly I'm not expecting Roma to lose tomorrow. Most people would favour Leverkusen but I still think Roma won't lose home in the 1st leg. De Rossi's arrival has started to change things for Roma. They are stronger and especially in home matches I have really liked their performance so far. Even against Xabi Alonso's team I think De Rossi won't be losing already.
But for the second leg I wouldn't have the same opinion. I already favour Leverkusen to advance to the final. But I just expect a tough challenge before that.

There's a possibility Roma will lose, just like Bologna humiliating Roma in Stadio Olimpico. Bologna is a rising star team similar to Leverkusen, it's either Roma will lose or draw, high unlikely they will beat Leverkusen when they always evade to lose.
Against Leverkusen on the third of May, I think Roma is definitely going to feel the pressure. Because they also have to care about the Serie A as well. But I guess it is not going to be that much of a problem as it is the semifinals of the Europa League competition, they will have two matches to actually have a good aggregate score. I would definitely be lying if I said that Leverkusen is not the better team going into this match. They are definitely the better opponent and as they also do not have to care about the Bundesliga anymore, I think they can put all their efforts and resources into winning the Europa League now.
People might say that it is important for them to keep a lossless streak in the Bundesliga. But I think winning the Europa League competition is more important than that. But on the other hand, Roma will have to worry about the Italian they lose too many matches in the Italian league, they will finish in the sixth position and probably not take part in the Europa League or any European competition. Roma might think that they will try to win the Europa League competition and not care about the Italian league. In that way, they will play in the Europa League anyway. But I don’t think that is a realistic plan to have