Martin didn't completely dominate so he went out of favour with Zidane in less than no time. The games he's played he's been either good, or just ok. He hasnt been bad, just lacked a standout performance. Not that any other Real players have been that amazing this season either, they are sorely lacking players with the Ronaldo factor.
But, that being said, Zidane is completely shafting him here. He wanted to remain in Sociedad for another year, Real took him back, and he's being repaid by being completely dropped after doing nothing wrong.
Ødegaard was the stand out player in La Liga last season, to be dropped for a B team player in a cup game is just beyond absurd.
Manchester United should strongly consider taking a punt here.