The Ajax coach will replace Rangnick at the end of the season.
After victory over Brentford, the German said: "So far we haven't been able to speak.
“Erik's focus is on winning the Dutch league, our focus was on our games. It's obvious quite a few players will leave and there is a need for top-quality players.
“I strongly believe that if everyone works together we can bring Man Utd 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."