This season, Athletic Bilbao is going through a good phase, in the last La Liga game they won and scored 7 goals and currently occupy the 4th position which would allow them to play in the European Champions League, so I see Athletic Bilbao with a greater chance of beating Roma who, despite being recovering in Serie A, are having serious problems in being able to occupy a place in the conference league.
I quite understand the point you're trying to make but sometimes, the performance of clubs in their domestic leagues doesn't have much impact in that of their European campaign. We've seen clubs like Sevilla win the the UEFA Europa League competition in seasons they were having a poor campaign in the Spanish La Liga. The Roma vs Athletic Bilbao round of 16 tie I think is one that we can't be too sure in predicting. Personally, I think the Italian Serie A side will get the better of their Spanish counterparts in that tie.