I know Rabiot since his debut, his evolution has been monitored cautiously.
He is very calm on the pitch (sometimes too much) and he manages pressure very well. His technique is beyond the rest, mixed with power due to his physicality. he can avoid tricky situations with ease, and he can do fast runs with the ball, good passes (not long range) and he is not bad when it comes to shoot. BUT, that palette is rarely visible full time during a game, and his seasons always been inconsistant. He likes to shine in big games, when he raises his level he can be among the top 3 players on the pitch (UCL, FRANCE). He his rarely injuried.
In the Man.U context he will need confidence from his manager, he is not the type of guy taking shit to be at his top. His weak point is his mental, sometimes we wonder if he really likes football (he declared that he almost never watch a game on TV so his passion must be elsewhere). I don't see him transcend himself just because he departs from Turin but maybe he can bring something more that what you already have in the midfield. In terms of power he can manage the PL that's for sure. Let's see if he regains some motivation here.