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.