Lukaku completed his move from Inter Milan this week.
Tuchel said: "We expect him to have an impact. Let's see where the impact is.
"We expect goals and he expects goals because he delivered goals everywhere he played. We will see how far the impact goes. He is big personality in the dressing room but I am absolutely convinced he will have a positive impact on Timo Werner, Kai Havterz, Christian Pulisic and the guys that play around him.
"We demand and he demands goals. We will set a clear target later in the season."
He added: "Is Romelu the missing piece? We hope, but at the same time we try to find the solutions for any questions asked in games. We want to still be a strong squad that comes form a team effort and Romelu is that kind of personality and profile that we were missing up front."