He can prove that his posture does not make him a bad defender because in terms of his actual posture he is still quite small for the size of a defender.
On the other hand, in every game he always makes a good impression in my opinion and this clearly makes Manchester United quite benefited because his presence is one of the good things to fill problems in defense.
Lisandro Martinez is Erik ten Hag's favorite for defense since his arrival from Ajax some time ago
Despite his small stature, his aggressiveness on the field is unquestionable, as seen from a total of 12 Premier League matches, Martinez get at least 3 yellow cards followed by 2 yellow cards in European leagues.
He's aggressive enough to stop enemy attacks, that's something defenders should have
He was a trusted figure for Ten Hag when he was at Ajax, so it is not too wrong to trust Martinez at Manchester United's defense at this time.
To be honest, a yellow card was the last thing to do because seeing the conditions in which the defenders fought, it was clear that their aggressiveness had to be strengthened. Not that in this case getting a yellow card is a good thing, but indeed I also can't blame when the defenders get cards.
Lisandro Martinez seem to have gotten favour from Erik Ten Hag since his arrival, despite his small stature he has been very physical and strong in the the defense for Manchester United probably one of the best in the team now. His aggressiveness is definitely paying off as it has helped in several cases to fend off opposition attacks. I really can't blame defenders for getting yellow cards because when you play aggressively you are probable to end up getting cards.