If you go back in the EPL history you'll have the first 'wave' of Serie A strikers (Bergkamp, Zola, Vialli, Ravanelli, Di Canio) did very well only to be followed by almost nothing. Sheva, Balotelli, Cutrone, Kean, Scamacca, Zaza, Ventola....they all failed. Hojlund did reasonably well in the Serie A, we know how he's fairing here same as Zirkzee. Is this down to United being shit? Well not really. Chiesa is struggling with Liverpool as well.
The Serie A is less physical and played at much lower tempo then the EPL. Someone with a strong physique could do an Asterix/Obelix in the Serie A. Just ask Lukaku and McT on that. However they would hit a brick wall in the EPL as its filled with the likes of Maguire, Van Dijk and van de Ven who could go pound by pound with any brute.
Capello summarized up perfectly for Zirkzee
“Go watch Zirkzee play in the Premier League and tell me if he seems like the same player we admired in Bologna – in Italy, he did what he wanted. “In England, he doesn’t even have time to think about the play before the ball is stolen from him. More or less the same thing happens to our teams.”