fabinho is a shiny new toy coming from monaco, who were the "hipster" team last season, but I would argue mostly due to a brilliant team set-up. mbappe, lemar and bernado were in my opinion the best individual players. falcao, glik and mountinho offered the necessary experience while mendy, sidibe, bakayoko and fabinho are very good players, but highly benefit from the coherent strategy which allows then to perform according to their strengths.
its kind of like s many dortmund players can not reach their level ioutside of the club. hummels and lewandowski had the necessary individual class, but kagawa, mkhitaryan or sahin benefited greatly from the overall team set-up. same holds true for dembele, who is a genuine world-class player opposed to aubameyang, who I predict will not shine in other clubs as he does at dortmund.
this video shows kagawa against monaco in both games. the holding defenders were fabinho and moutinho in the first match (the one right after the terrorattack) and bakayoko with mouthinho. I picked this one, because most of you guys saw kagawa play multiple times and can judge his quality fairly, eventhough moyes played him as a winger.
https://www.you-tube.com/watch?v=zZrIKa6-Guw
The point I am trying to make is, that fabinho is a very good player, but we will not be the strategic DM the system of mourinho required. like kagawa could not deliver while he was at united, he matches the quality of fabinho, while both play at team who are set-up according to their playstyles. monaco came from quick transitions from somewhat chaotic ball winning opportunities, those will never happen under mourinho in the PL. fabinho is very quick in anticipating those and move the ball forward, but i can not see him fulfilling the DM-role and Man Utd on the highest level. he would be better at arsenal or even madrid (like casemiro).