Instances where Xavi's Barca played against a back 5, or a variant with a 3 (Using sofascore data for this)
1. Benfica 0-0 (3-4-2-1)
2. Frankfurt 1-1 (3-5-2)
3. Frankfurt 2-3 (3-5-2)
4. Mallorca 2-1 (3-4-1-2)
5. Getafe 0-0 (5-3-2)
6. Mallorca 1-0 (5-4-1)
7. Inter 0-1 (3-5-2)
That is 2W, 3D and 2L out of 7 games, with 6 goals scored. And in the 2 wins against Mallorca we were very lucky not to drop points either time, in fact they outplayed us last game and probably deserved to win.
As you can see we don't come up against 5 ATB much in LaLiga, as most of the teams play 4-4-2 or 4-2-3-1, but when we do our offense is silenced and we don't score. And teams in Europe are using the 5 atb very frequently against us and will continue to on the evidence of this performance.
5 at the back kills Xavi's wing play system and it's likely we will need to develop a new tactic/change tactic soon or we will soon be figured out completely when the stubborn Spanish coaches finally twig on.
Further xG analysis :
Benfica 1.06 - 1.4 Barca
Frankfurt 1.32 - 0.6 Barca
Frankfurt 1.67 - 2.82 Barca
Mallorca 1.46 - 1.20 Barca
Getafe 0.45 - 0.20 Barca
Mallorca 1.08 - 0.7 Barca
Inter 0.16 - 0.52 Barca
In these 7 games against 5 ATB, we created 7.44 xG and conceded 7.2 xG, leaving a marginal positive of +0.24 xG over the games against significantly inferior teams.
We were outcreated in 4/7 games and in 4/7 games we didn't even manage to create 1 xG, with 2 of those being this season.
TLDR - When our opponents use 5 atb we can't exert any dominance at all and in fact often get outplayed and lose the games on xG, even against much worse teams such as Getafe or Mallorca.