Sure, his honeymoon phase will be probably even more impressive than Bellingham's. But I'll wait till the end of next season to rate him. Bellingham also stopped his Ballon D'Or performances along the way. There comes always a time, where it's not that exciting anymore to wake up to the same face
If he comes here(i hope not), he will no doubt score a bunch of goals, but i don't particulary care for that, when he will push Vinicius out of the team.
Also, imagine the atmosphere in the dressing room, when Mbappe comes in and earning twice of some of our key players in UCL-wins, when he has none and is new at the club, friendship FC no more.