The January transfer window must be used to rebuild the squad to get them back to their best form. As a coach who has just set foot in the EPL, Amorim must be able to show what he has done with his previous team. Maybe he will bring in some of his former subordinates at Sporting to make the job easier at Manchester United.
He is already aware about how competitive the league is because he has played against many teams that he couldn't beat from Premier League in another leagues, he even met with Manchester United twice with his sporting teams twice and he couldn't beat them. The thing is that Arteta was more like pro of Guardiola, he didn't score make his boys to score the usual ways and that's how Arsenal was able to bypass their defenders without counter attack.
Amorim is a new coach and will adapt just like the other guys succeeded here. Liverpool is one of the best that has improved in performance but he didn't buy and sell players as many people thinks and yet they are doing fantastically in any games they played. The coaches can only do their best but the main result is from the effort of the players, you can't tell me that Guardiola isn't a great manager but yet the players are fumbling everyday losing matches in the last few weeks.