He was our focal point, our best finisher by some margin which is why he was involved in so many of our goals.
In Utd's squad, with the likes of Pogba, Rashford, Mata, Mikitarian and others who all have good shots and can create chances, I doubt Rom will get the same goal tallies as he got with us.
Look at Rooney, only twice in 13 seasons did he get over 20 goals in the league in a season. It's not the United way to have one player score all the goals. Of course, most of those were playing Ferguson's way so under Mourinho it could be different.
He'll still win stuff, but I doubt he'll win the golden boot any time soon.