Christian Pulisic will miss Chelsea's Premier League trip to Arsenal after testing positive for Covid-19.
The USA forward missed Chelsea's open training session on Wednesday, and is not available for Sunday's game at the Emirates Stadium due to self-isolation.
Hakim Ziyech and N'Golo Kante are both back in training, and could yet prove their fitness in time to feature in the north London derby.
"Christian's absence is unfortunately pretty easy to explain, he had a positive test, and needs to follow the protocols," said Tuchel.
"So he was not in training and is not available for the game."
Ziyech suffered a nasty-looking shoulder injury in Chelsea's penalty shoot-out European Super Cup win over Villarreal, while Kante has been out of action with an ankle issue.
But both influential Blues players will now have the chance to prove their match fitness in Saturday's training session ahead of the Arsenal clash.
Chelsea opened their new Premier League campaign with a resounding 3-0 home win over Crystal Palace last weekend, and will be hoping to extend that impressive start.
"Hakim and N'Golo were both in training today, and did everything to be in training today and had absolutely no problems," Tuchel added.
"But they need another more physical, intense training session tomorrow with the team.
"We can decide after that, but right now it seems very positive that both can be in the squad for Sunday."
