The Croatian has been injured since the quarter-final against Porto last month.
Kovacic had returned to training recently, but was not fit enough to face Real.
The tie sits at 1-1 ahead of the clash at Stamford Bridge.
"I can give you the status right now which is that Mateo will not be in the squad," Tuchel told reporters.
"I think Toni will be fine to play with a mask and he trained with us yesterday. Everyone else trained yesterday too and will be out training with us later today.
"Hopefully we will have no new injuries from today's training, so that means everyone is available for the match except for Mateo Kovacic."