The Danish centre-half made himself unavailable for selection for the FA Cup final last weekend.
Christensen was meant to start the game, but felt he was not fit enough to play a part in the eventual loss on penalties to Liverpool.
Tuchel said to reporter: "Andreas came the morning of the match to tell me he's not ready.
"He had his reasons, they stay private and confidential. It was not the first time as you can see. Over the last weeks that we had the same situations.
"We thought that we are in a good progression. He played very strong in the match before the final. The conversation took place and we have to respect it. He still has our support but for tomorrow we're not sure."
"I don't think that it's that he does not want to play. There is a big difference," he added.
"I don't know the reason but he is always very nervous. I don't know what to say. We had to accept it and we will accept it.
"I try the best I can to not take things personally. I still believe that Andreas wanted to play and I still believe that he could.
"He struggled over the last weeks as you can see when he was not in the squad, although he was physically available. So that was not completely out of the blue sky.
"It was not the ideal situation for us because we thought we had prepared him in the best way possible in the game against Leeds. He is a key player and if you arrive in a final you want your best players available and this was not the case."