Antonio Rudiger has signed a four-year contract with Real Madrid.
The 29-year-old will end his five-year spell with the Blues this summer once the season is over.
Andreas Christensen will also move to Barcelona on a free transfer and Chelsea are losing two important centre-backs in just one window.
But Tuchel wants the duo to complete the campaign on a high.
Tuchel said: “I’m absolutely convinced that they want that. Maybe they underestimated the impact that decisions to sign for another club have, the impact that it has.
“I have the feeling that they experience it now themselves, but that they don’t want this and they don’t do it intentionally, but this is the case, we feel that it is different, a different approach to the games.
“Maybe these kind of shock experiences are necessary to really wake up and not go out of the season like this.
“Toni (Rudiger) is important. You never finish with development and this is a key moment for his personal career. He knows it and I think he will step up. It is the moment to do it.”