Speaking to Sky Sports after the Brentford win on Monday, Ralf Rangnick said: "It's obvious quite a few players will leave [this summer].
"There is a need for top-quality players. I strongly believe that if everyone works together we can bring Manchester United back to where we need to be.
"Other clubs only needed two or three transfer windows to challenge but from now on we need to bring in top-quality players who really help raise the level.
"If this happens then I don't think it should take too long."
Rangnick will be transitioning into a two-year consultancy role at Man United next month in addition to taking over as the new Austria national team head coach.
Erik ten Hag is set to replace the German in the Old Trafford dugout and the Red Devils' chiefs are expected to back the new manager in the transfer market.
