Reality is none of us know where ETH will play Martinez if we sign him, just because ETH moved Martinez to centre back for Ajax doesn’t mean he doesn’t think he can’t play there for United as ETH is very much known for his tactical flexibility and seeing players in different roles to suit what he wants to do.
I’m 99% certain that Martinez was always a defensive midfielder until moving to Ajax although I could be wrong ? Having an aggressive, heavy pressing, high stamina ball winner who can pass like Martinez in midfield with De Jong would be about as perfectly balanced as we’ve had since Scholes and Carrick 15 years ago.
Some players are naturally brilliant in more than role as I remember Marcel Desailly playing centre back for France but midfield for Milan then there’s Matthias Sammer and more recently Joshua Kimmich so ETH may have seen Martinez as a centre back in Eredivisie but more suited to midfield in the PL.
If Varane is fit and ETH has confirmed Maguire is captain then I’d imagine they’d be first choice although ETH is so tactically flexible and adapts that he could sometimes go 3 at the back or in certain games at home use Martinez as a ball playing centre back when we need to force the game but then Varane/Maguire and De Jong/Martinez as his centre back/centre midfield pairings are solid.