Goals from Hassane Kamara and Boulaye Dia secured a surprise victory for Reims at the Parc des Princes on Thursday (AEST) as PSG slipped to its second defeat of the season.
PSG remains top of the table after seven matches but is only ahead of second-placed Angers on goal difference.
The underwhelming manner of the defeat was what irked Tuchel as they lost at home in Ligue 1 for the first time since May 2018 – a run of 22 matches.
"I'm disappointed with our performance," Tuchel said.
"Playing in the Parc with the Parisian jersey is always special, and it is true that the performance was insufficient.
"The first 10, maybe 15, minutes went well but then every minute that passed was worse and worse.
"We did not show what we are capable of doing – so defeat is deserved."
PSG were without the attacking qualities of Julian Draxler, Edinson Cavani, Kylian Mbappe and the on-loan Mauro Icardi – all absent through injury – while Marco Verratti was suspended.
Tuchel made five changes to his starting line-up as he rotated his injury-hit squad and PSG lacked rhythm and creativity with much of the impetus in the final third placed on Neymar's shoulders.
The Brazil forward, who had scored the winning goal in consecutive 1-0 wins in Ligue 1 for PSG, was not able to perform the feat again.
Tuchel, though, defended his decision to make so many changes to the team which started against Reims, despite the result.
"I regret nothing," he added. "Because I have to manage my team and we lack players and we cannot use too few. And it's easier to analyse afterwards.
"We have to make decisions and I have made them. Now these are the consequences, but I do not think we lost because we changed players too much. It's more due to the way we played.
"We lost logically because Reims was closer to its optimal shape than we were.
"I'm disappointed with our performance, but we are always together and we remain united in defeat."