what's the fixation with Tah Fuck him. 60 % of our problems are a back up LB and 40% a very good striker instead of Ferran.
Most competent fullbacks aren't much better defenders than Martin is. There is a reason why guys like Pep & Arteta uses 4 CB defense.
A lot of current generation of fullbackss were developed as wingers/midfielders and moved to fullbacks, even the more natural one had the idea of "inverted " fullbacks as they were developing as players.
And CB has to cover for the defenders around them, and GK has to cover for the entire team defensive shortcomings etc.
I want an upgrade over Martin, but for me it is delusional to think that another "backup level LB" would have made the game that much better defensively than a better CB.
There is a reaaon why no Fullback has never made it to ballon d'or, the only positions to not to, and that CB are more expensive than fullbacks. Elite CB is far more transformative than any fullback signing.
In a way, you have a valid argument, Martin is a bigger weakness than any CB, that is right.
But a great CB will solve far more problems than any LB signing could do.