Thomas Tuchel says Ben Chilwell's disappointment at being dropped by England has been a factor in his recent absence from the Chelsea team.
Chilwell went into this summer's European Championship finals on a high, having played the full match as Chelsea beat Manchester City in the Champions League final in Porto in May.
However, he did not play a single minute at the Euros, with Gareth Southgate preferring to use Luke Shaw and Kieran Trippier - a right-back - on the left of his defence.
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Chilwell returned from his post-Euros holiday to find his place in the Chelsea team had been taken by Marcos Alonso, and he was handed further disappointment when Southgate dropped him from his squad entirely for England's World Cup Qualifiers earlier this month.
It means Chilwell's most recent competitive football was that Champions League final nearly four months ago, and when asked whether being snubbed by England was tough for the left-back to take, Tuchel said: "Yes, that has absolutely been difficult for him, and I think it's pretty normal.
"He came from a Champions League win, 90 minutes, a strong performance and a good end to the season in general. He played a lot of minutes for us in many important games, at a high level.
"Then it was individually for him a tough Euros, because there were chances where he thought he could have played. I know it was a tough one, because you feel a part of the team, but you don't feel it really if you don't sweat it out on the pitch and if you never wear the shirt. It's a strange feeling, also to keep training for four or five weeks continuously.
"He had a break and it was hard for him to totally relax and get this disappointment off his shoulders. When he arrived here, I felt him a bit mentally tired, still worrying about the situation.
"At the same time, he was very unlucky because Marcos was here the whole pre-season and he took this opportunity with such a huge quality and attitude. Marcos was crucial for us from the first day in training and then he delivered in Chilly's position."
Tuchel revealed he held talks with Chilwell to ease any concerns the 24-year-old may have over his playing time, and suggested he could make his first appearance of the season when Chelsea host Zenit in their Champions League opener on Tuesday.
"We had some talks with Chilly about the situation," explained Tuchel. "He knows I would have done the same thing if it was the other way around - to trust him if he was here and showed personality and attitude like Marcos did.
"There are no further worries for Chilly. He just needed to be a bit patient and keep training. It took some time - we had to give him a little bit of time to be more patient with himself, to find the joy again and to find things a little bit more easy on the pitch and to not want to force it too hard.
"But he's in the race now. He's in good shape so he can play any minute."
Tuchel: I have nothing to prove
Tuchel took charge of Chelsea last season after predecessor Frank Lampard had already steered them into the Champions League knockout stage, but the German batted away suggestions May's final victory was somehow diminished by his late arrival.
He said: "It was never a problem for me to accept the work from Frank that he did throughout the group stage.
"But it still feels like my title - it does not feel like half a Champions League that I won. In the moment of the final I was responsible, and during the knockout games, so it felt like my team and that we were together.
"Do I have something to prove, that I can bring teams through the group stage? No. This has nothing to do with it.
"We want to win it again but we need to accept how tough this is. Nobody is shy here to admit that we fight for any competition we play in. We want to have a good start tomorrow and we need to start from scratch."
Tuchel confirmed Chelsea will be without Christian Pulisic and N'Golo Kante for the visit of Zenit.
Pulisic is yet to return to training after being injured on international duty with the USA. Kante returned to training this week, but the head coach said he would miss the midweek fixture so he is "absolutely ready" for the trip to rivals Tottenham on Sunday, live on Sky Sports.