Kepa was ditched by former boss Frank Lampard but Tuchel believes his world-record fee is a huge burden.
Tuchel said: “From outside we had a view on his situation and some difficulties on the sporting side.
"Maybe the circumstances of his transfer are a little bit on his shoulders. That was the impression from outside.
"From inside, I see an open personality. A hard working guy with a lot of quality. He deserves to be here.
“The good thing for him is that we start new, that this is a new start for him because I don't have a history with him.
“He does everything to impress in training and this is the only thing he could do.
"It is a special position, it has its own rules. If you make mistakes, everybody sees it, talks about it so you need a lot of mental strength and quality.
"Kepa gets the chance tomorrow and I hope he can show what he shows in training."
Kepa has been in limbo with a huge fee and wages putting off rival clubs to buy him and end his misery - while Chelsea brought in Edouard Mendy to replace him.
With Euro 2020 this summer his place as his country's number one is in serious jeopardy.
Tuchel said: “I want him to play a normal match.
"He does not have to show me or anybody else that he is ready to be Spain's No1 in the summer.
"He has just to show his quality. Not more and not less.
“We have to manage our own expectations because I don't expect him to score and to play with a clean sheet.
"I just expect he is there and he helps us with the quality he has and gives a solid performance to help the team."