What Flick managed to do with this player... is absolutely incredible
And this is not hate on Martin. Improvement could not be possible if he himself didn't commit to it as well
But he is currently couple levels above what he was when saw him first time last season
So many managers would just scream they want a new LB and CB while this guy managed to create a serviceable player out of seemingly devoid of talent dude who was playing in third division well in his twenties
One of the things people miss is playing with much better players will raise your level, even if you are a second division player originally.
Your team will help you a lot, as an example, the quality of passing is better, the instructions are better, the players in front of you are better, and it is easy to fit in as long as you are not throwing or making major mistakes.
To play in Barcelona will raise the level of any Segunda player very very quickly. Martin has probably learnt enormous amounts from his teammates and the coaching staff.
We see this example with the likes of Mingueza, or Araujo, who were not overly impressive for the B team. But playing for a top team can shield your weaknesses especially if you are playing in the LaLiga games where the team will have most of the ball and you won't face many attacks.