I do believe that "he was world class at Juve" is less a function of his brilliance, and more a function of the fact that Juve won the league. Juve would have won the league without him too, in my opinion.
I just want to leave some stats here.
Juventus - 4 seasons (2012-2016) - 166 apps (142 starts) - 32 goals, 27 assists
Manutd - 4 seasons (2016-2020) - 165 apps (144 starts) - 32 goals 32 assists
His output at both Juve and Utd seems similar. Fbref doesn't have his Juve stats but if I find them I will do some work to figure out how his passing / play at Juve compares to his passing / play at Manchester United, but I suspect it would be similar.
The thing is though, we got Pogba when he was supposed to be at his peak, and we should be seeing better numbers, not similar to what he was doing at Juve.