Board Thread:Versus Debates/@comment-31338621-20200309130706/@comment-31338621-20200328183717

It's probably him noticing the sweat on Sasuke's forehead or Sasuke's hand coming to his eye.

If it was just a second like usual, with Max time dilation, it's unlikely Zetsu would have noticed the illusion, during the middle of the illusion. Tsukuyomi doesn't have to last only a second, the time is variable.