Real have won the competition in the last three seasons, but stumbled out in Russia on match day two on Tuesday, losing 1-0 to CSKA Moscow.
The Champions League holders have failed to score in three successive games, as the pressure on manager Julen Lopetegui rises.
Karim Benzema's early season form has already faded, while Mariano Diaz has been eased back into the first-team since his off-season return from Lyon.
And goalkeeper Navas has made a brutally honest admission after the surprise result in Russia.
"Cristiano left the bar very high at Real Madrid," Navas said, as quoted by Eurosport. "You cannot cover the sun with a finger.
"He made many goals while he was here, but it's already the past and we cannot live in the past."
