I listened to a podcast and one of the analysis suggest that their momentum just got fucked over by their failure to participate in the UCL this season. It's a mentality thing, and they didn't really do a good job of rejuvenating their forwards/attacking players either. I think that makes sense. You need time to recover after a terrible result, especially when the disappointment is so big. It's not as easy as saying "let's do it again", even if you're a pro athlete.
It is not reasonable for players to just give up simply because they ended up not getting to play in UCL, that is not reasonable. I understand not being great at the moment, but that is not something they should be accepting all season long. They need to do better and I sure do hope that they do, because that team definitely deserves to stay in the league.
Worst case scenario they will have a bad season, finish like 14th or something for example, not a great year, but at least stay and be good. However, if something major happens, and they just fail to gain any good momentum at all, and end up with bad results for a long time? That is not going to be ok at all, they are going to get relegated and that's not fine.