Tuchel opted to start Timo Werner ahead of Lukaku as Chelsea defeated Crystal Palace 2-0 at Wembley Stadium.
Two second-half goals from Ruben Loftus-Cheek and Mason Mount gave Chelsea the victory against Palace.
Explaining why he did not start Lukaku, Tuchel said during his press conference: “He is not fit enough for the intensity of our game in an FA Cup semi-final.
“It is not easy to come into a game such as this with so much riding on it after a spell out with injury.
“It is just a case of him not having as much minutes recently as the other players, his match fitness is not quite there otherwise, there is a big chance that he would have started today.”