In Manchester United - Tottenham 0-3, Spurs had 5 players in the Starting XI signed from Serie A: Vicario (Empoli), Romero (Atalanta), Udogie (Udinese), Bentancur (Juventus), Kulusevski (Juventus) so it's not really a problem which league or club you buy from..
Manchester United signed top players from top clubs as the likes of Schweinsteiger, Casemiro, Di Maria, Alexis Sanchez, Varane, Mason Mount, Sancho and no one of them did really work out.
I think simply is a problem of managment if Postecoglu has been great at bringing out the best from his Serie A signigns while Ten Hag can't do the same with Onana, Hojlund, Amad, Zirkzee (+ Amrabat last season)