#1 United do not have too many No. 9s
United only have Romelu Lukaku to lead the line as a proper No. 9 right now. Even though Rashford and Martial are both capable, neither of them have shown any reliability as a main striker, preferring instead to run into the box from the wings. Sanchez was phenomenal leading the line in the preseason, but he does not have the physical strength to bully defenders and even he is more effective from the left.
Zlatan, in such a case, would be just what the doctor ordered for Manchester United. Zlatan was only the 4th player to score 14 goals in his first 20 Premier League games, joining a elite group of strikers. He was the 4th quickest to 15 Premier League goals for Manchester United.
He played a total of 748 club games in his career, scoring 436 goals. He scored another 62 goals for Sweden in international games. United would not get a better back up to Lukaku in the market at present.