Because at Shakhtar his job was to sit in front of the back 4 with another DM but he was the more attack minded one showing a good eye for through balls and helping the team to transition quickly from defence to attack. I imagine he would be top class with a player like Kante for example.
At United he plays as number 8 and has to create more. It's not really his game. City use D Silva in the same position.
Its correct that Fred played in a double pivot at Shakhtar.
His partner was almost always Taras Stepanenko, Shakhtar´s captain. And its also correct that Stepanenko was the more defensive-minded, but there was not much of a difference.
The player stylistically (and talent-wise) closest to Stepanenko in todays United squad is probably Herrera.
I believe Fred suffers in the same way as Pogba playing as a #8 in a defensive-oriented 4-3-3 as Mourinho`s is.
They are both best on the ball, with plenty of options in front of them, which is just not happening for us playing a counterattacking system as we do now.
The comparison to David Silva is interesting, I think Fred would do well there for City and that is probably the role Guardiola wanted him to compete for. But playing as a #8 for Pep and playing #8 for Mourinho is two completely different things. Non-comparable.
IMO we should switch back to a double pivot and have Pogba playing together with either Fred or Herrera. It is what suits all of those players the best in the defensive system as we play today. And it would still have a resemblance of a modern mobile CM-setup.
Matic needs to be a demoted to a squad player immediately.
Play Lingard as #10 or even Mata or Fellaini depending on the opposition, and we would actually be using our players to their strenghts.
IMO.