I think our issues at fullback are overblown. Depth killed us due to injuries... we can sub in Casado and Bernal, fit Lewy, a potential LW signing and suddenly we are a 50-50 tie away from the CL. Not even adressing our two big injuries in defence - heck, if Kounde is fit and we can use Garcia as a DM sub... all around just unlucky. PSG and Inter just didn't have the same struggles this time.
Injuries happen. They're likely to cause issues next season too. But let's focus on FBs for now. Before the injury, you had Kounde with most minutes played for club and country in Europe. He's a resilient guy unlike Balde.
Balde's injury history:
24 / 25 8 games 30 days
23 / 24 23 games 196 days
22 / 23 3 games 62 days
21 / 22 16 games 92 days
20 / 21 16 games 66 days
So it's bound to happen again and we're forced to rely on Martin. Which is a huge drop.
Not to mention you cannot rely on him to relieve Balde and give him some proper rest.
I have trust issues re Eric as our backup RB, but that's me.
And I firmly believe you can improve depth the easiest by signing first 11 players (eg our two easiest to improve positions like a new striker and goalkeeper - though the latter is a special case).
Ideally, but given our situation, as long as we have Mats and Tek that is Lewa and Ferran we're not making changes in those positions. Personally think that a new GK and a world class CB would have the biggest effect.
We sorely need depth. A forward, one or two FBs and we'd be in a much better position.