Bakayoko reminds me a lot of Matuidi, who I believe Mourinho was interested in during the summer so this move makes a lot of sense. He seems to have good composure and confidence in using his considerable frame to retain possession, similar to Pogba and Matuidi. He also drives forward like those two, has the defensive box to box qualities of Matuidi and the stamina to match.
However, he will still struggle in a double pivot with Pogba, and we will have similar problems to that of the French national team. He, like Matuidi and Pogba, lacks the positional discipline and would be more suited to an additional holding player (i.e Kante, Fabinho). For Castles to suggest that he is a #6 is a bit of a surprise to me, he doesn't seem that player. Herrera is the only one capable of the discipline required to replace Carrick, Blind notwithstanding.
For what it's worth, I'd like to see:
Timothy Fosu-Mensah played primarily as a centre back and also as cover for full back.
Daley Blind played as a direct replacement for Carrick in games where we are expected to dominate. Ander Herrera as the other option for a holding midfielder. Bakayoko of Fosu-Mensah could deputize in games where we need a physical presence.
Pogba, Bakayoko/Niguiz and Herrera to rotate as the two players in front of the holding midfielder.