I watched Monaco all season. He is not a DM, or he doesn't play like one. He won't sit, his passing is ok and he is decent under pressure but he loves to go for a wonder and break forward. If you want someone to anchor a midfield he is not your man.
He is better at getting back at people as he uses his athletic ability and surprised strength but it's chasing back at people and if you've got Pogba wanting to get forward and make something happen (which coincidentally is the same thing Fabinho wants to do)
Also I noticed people keep referencing his goal tally with things like from a defensive midfield position. He scored 2 goals from open play and the rest were penalties. 9 out of 11, all pens.
As for laughing off the Matic, ideally he wouldn't be my first choice, but it's a player like him that United need for either system. Again Fabinho is not that guy that will sit and anchor a midfield. He doesn't read a game well and is positionally suspect.
Who would be?
I agree we need a disciplined midfield anchor.
Edit: seen your response above.