I only saw him before Ajax, but I'm not surprised that he's doing well. Looked a quality ball playing CB in Argentina
Aye, what really stands out is that he can distribute/recycle to a reasonably good degree (#7 in passing progressions in the Primera División and the most passes of any defender)
and dribble with the ball (#3 in carrying progressions in the Primera División). Peak Matić (not the old cumbersome version) had that quality, too — which made him more than just a basic destroyer or passer from the deep...more of a considerable box-to-box threat with line-breaking abilities, and frequently created mismatches in the heart of the pitch when the opposition wasn't paying attention to him (especially if they were playing with a lackadaisical #10 who couldn't/wouldn't press with intent or track his runs).
When you consider Martínez's versatility/adaptability on top of those ball carrying and passing and line-splitting qualities in transitions: as you said a quality centerback for his age (might not have a long-term future there in an environment like the Premier League because he's not the tallest at 5'10", unless he's LCB in a back-3), brought in as potential backup for Tagliafico at leftback, and now performing at a solid level as a midfielder — he becomes an obvious target for United because we need help in at least two of those positions. Definitely hope we're monitoring him because he's receiving good education at Ajax, will become a more compact player who's more aware of space and organisation, and fits the modern press-resistant style...slightly hot-headed at times, but that youthful aggression can be channeled in a positive direction.
Their other centerback/defensive midfielder hybrid, Edson Álvarez (signed from Mexico and compared with Márquez) is quite interesting, too — no one could hope to fill the void created by De Jong's departure because he was unique with his unmatched instinct for deep progressions and carrying the ball up the pitch in effortless fashion as the team's steering wheel, but Overmars and Veldmate have done quite well with these signings for just €23 million.