The Spain number one has been sidelined with a broken hand since January 23 but after returning to training was tipped to be back in the side for the Round of 16 second leg at Old Trafford.
However, Mourinho has confirmed that Casillas will play no part as the Spanish champions look to better the 1-1 draw they recorded against United at home last month.
"Everyone is fit and ready to play and be considered, with the exception of Iker Casillas," Mourinho said.
"We are sorry we had to travel yesterday (because of a baggage handlers strike) because we don't like to wait.
"The emotion is high and we want to play.
"Tomorrow the world will stop to watch this tie, which doesn't look like a tie it looks like a final."