Martinez is ace and can play as a deep lying midfielder. Too small to be a competitive CB in the Premier League though, teams would isolate him aerially and just bombard them. He's played mainly in a 4, in fact never in a three but has always been in a side that constantly has the ball. Just doesn't seem like he'd be the best signing given what they need - he is in the bottom 23% for winning and challenging for aerial balls in Europe, imagine the Utd fan's reactions when he gets bullied and jumped on for two headers by Ashley fucking Barnes next season.
One of those players you can simultaneously appreciate but know isn't the right profile. As a Masch-lite style ball winner and recycler in midfield, perhaps. Great tackler, decent presser, great passer. Centre Half? No. Too small, too used to playing a system that protects him from facing what he's poor at. He's also spent the last three months out injured and isn't expected to be fit until the new season, which doesn't help, having time at a new club be rehab time.
Edson Alvarez is the Ajax player they should be all over as a DM, a lightbulb moment. Can do all the nasty stuff but can play too.