I've been trying to free myself from the thought that we should replace Matic like for like. As far as i can see, the deepest midfield position is a role we've changed between "generations" at United previously. We didn't directly replace Keane, we brought in Carrick instead, a quite different player type. We didn't quite directly replace Carrick, at least not initially. He was, rather, phased out, and we had sligthly different players instead ( Schneiderlin(early attempt), Herrera(though different initially), then eventually a similar Matic)
Still, it might be different this time around. This time OGS inherited a squad, and is trying to form "his" optimal squad to fit his ideas. The question is if Matic is the "perfect" fit for what OGS wants for that role, or if he wants something quite different. This could be hard for us to anticipate atm, but i think this "Matic replacement" discussion should take this into consideration.
Personally i think what we need, to complement Pogba and Bruno, is someone with mainly defensive responsibilities, allowing Pogba more freedom to roam offensively along with Bruno. This requires an ability to "cover the counter" to an even better degree that Matic can today. Key attributes: Positioning, Tackling, Stamina, Awareness.
Also, he should be able to retain and redistribute the ball effectively. Key attributes: Passing, Technique, Press resistance, Awareness
Being a natural deep focal point of our team, he should be able to control the tempo: Key attributes: Awareness, passing range, press resistance.
I don't watch enough football outside United to have any meaningful evaluations of suitable candidates, but maybe the rest of you do? Maybe someone sees a quite different role being taken by the "Matic replacement"?