Lucas was called up consecutively last fall, he was also called up this season. Over his career, despite being 2 years younger than Costa he has twice as many caps and has been featured 20 more times across all age groups. To say he isn't competing for a spot is patently incorrect. It isn't even debatable. Is he a permanent fixture in the squad? No, but that designation is specific to Neymar.
In which case, Monaco had the best defensive record in last seasons UCL. Paris had the best defensive record through the group stages.
In this context PSG doesnt matter, because Lucas is playing FOR them, not against them. We were talking about the domestic opponents Costa and Moura are playing against.
Then there is only Monaco left, and the example you picked was from last season.
This year:
Bordeaux - EL (4th), most goals conceded in the group
Lyon - CL (4th), most goals conceded in the group
Marseille - EL (2nd), average number of goals conceded but in the easiest or 2nd easiest group (Group E & F are fighting for that title)
Monaco - EL (3rd), most goals conceded in the group
Saint Etienne - EL (2nd), that's fair they had a tougher group and just conceded an average number of goals conceded
PSG had the tougher group this season and last.
Not last season. They had Barcelona in it, but you need to finnish 2nd to qualify and the other 2 participants are called Ajax and APOEL. Bayern had more depth in their group with City, Roma and CSKA in their group and these are better teams than Ajax and APOEL.
This season their group was harder, but what I said was "has better CL matches against better opponents" and this true, because he played 5 matches from the start and rocked all of them, even the lost one to Arsenal, while Lucas played short caps. 2 of them also against Real Madrid, but that was a combined 39 minutes in which he didnt manage to contribute to something. Lucas CL highlight was the last game against Shakhtar, when PSG was already comfortably through,
fielding a B-team.
EDIT: I'm a bit tired of writing these long comments, discussing 5 subtopics at the same time with 2-3 different people from which someone branches off another subtopic and so on and so on. If this discussion continues, let's shorten it at least and get back to the elementary things.