The Manchester United defender missed selection for this past week's squad for World Cup qualifying wins against Albania and Latvia.
Tuchel admits the decision was made on form and not fitness.
He said, “It was a close decision.
“We chose Marc Guehi and Ezri Konsa at the moment. I think they are a little bit in a better rhythm and deserve to be with us.
“Two right footers in central defence and the rest were left footers in the centre of defence. It’s not so easy for him at the club at the moment. He plays in a back three on top of it.
“The second reason was that he was not fit when we did the nomination. It was presumed that he can play.
“But we didn’t want to nominate a player who we had to take a risk on and in the first training or second training we constantly need to assess whether he can do full training session or half training session.
“We did the same with Reece James and only because of the feedback that he could do every session full was the reason we brought him in to camp. Harry knows that.”
Tuchel also revealed: “But he’s a top professional and he’s very important for England. He took it unbelievably well. I spoke to all of them who were not on the list to give my direct feedback on my decision.
“He wrote me messages to wish us good luck and you see what a person he is. He is a top guy and he will always be in contention.”