The Serie A leaders were insipid at the Groupama Stadium and had no answer to Lucas Tousart's solitary strike in the 31st minute.
Cristiano Ronaldo huffed and puffed in attack, but was powerless to prevent Juve ending the game without having registered a single shot on target .
Sarri cut a frustrated figure and says he was stunned at how slowly his side moved the ball in comparison to how they did in training ahead of the game.
"It's difficult to explain why, but in the first half we moved the ball too slowly so the opposition were going to press, win the ball back and score," he told Sky Sport Italia .
"We lacked determination and aggression when attacking, and for 15 minutes lacked it defensively too.
"The second half was better, but it's not enough for a Champions League match. I don't know why I cannot get the players to understand the importance of moving the ball quickly.
"This is fundamental. We'll keep working on it and sooner or later this concept will get into their heads.
"I continued to tell them, and there were many who were doing it, moving the ball too slowly and therefore getting into the wrong positions.
"We had training yesterday and the ball was moving twice as quickly as it did tonight. This is the opposite of what should happen. Lately we're doing a lot of things opposite to in training.
"When the ball is slow, you lose positions, you allow the opposition to close you down, to be aggressive and steal the ball back.
"I don't even think the Lyon pressing was that strong. We just moved it slowly, never varied the tempo, and when you do that, it's unlikely you're going to create anything."
Juve defender Leonardo Bonucci gave a similar assessment of his side's performance , saying "something wasn't switched on" during the game.
The Serie A leaders will have the chance to atone for their errors in the return leg in Turin on March 17.