In order to get to the final, Roma must first beat Milan, and then the winner of the Bayer - West Ham pair (Bayer). I think the probability of this is no more than 10%. The problem for Rossi is that Mourinho has set the bar very high and even repeating it will be difficult. In general, everything can end as if there was no change of coach (if Milan knocks out Roma, and in Serie A Roma does not reach the top 4).
I am glad that at the moment the Bayer - Liverpool final is becoming more and more likely, although Bayer has a more difficult path.
They can beat Leverkusen again but beating AC Milan look to be impossible for Roma and De Rossi. AC Milan and AS Roma understand each other very well because they are both from Italy and playing against each other many times in Serie A. Roma historically don't have good head to head stats against AC Milan and it is not good signal for them before two quarter final matches with Milan in Europa League.
Playing against Leverkusen that is from Germany and Bundesliga can be easier for Roma if we exclude a fact that Leverkusen are stronger than themselves last season. Roma will have to pass through AC Milan before playing with Leverkusen and the first condition is very hard for them.
I don't know why you rely so much on history/previous games. It seems to me that if it comes to the game Bayer - Roma, the bookmakers will quote Bayer as 1.2-1.3, that is, as a clear favorite. As for the game Roma - Milan, yes, Roma’s chances are small here and in general I don’t think that it will be an interesting game (not in the sense of the process but as a pair) because when teams from the same country meet in European Cups and these are not pairs like Real - Barcelona, then these meetings turn into ordinary ones, which we see several times every year.