The reigning European champions were beaten 3-0 by Juventus in Turin on Tuesday, a result that means Chelsea can only qualify from Group F if Shakhtar Donetsk beat the Italian club in the final round of games in two weeks time.
Fabio Quagliarella, Arturo Vidal and Sebastian Giovinco were on target as Juventus moved to within one point of booking their spot in the last 16 and the Blues' goalkeeper was realistic regarding Chelsea's future chances.
Cech stressed: "We said before the game we had our destiny in our own hands but not any more.
"Now we have to do our bit to win our last game and hope destiny will be kind to us.
"Being realistic, Juventus played really well and they need a point. Shakhtar have already qualified so they don't need to take a chance on the result.
"It is a very sad moment because last season the Champions League was like a dream. I wouldn't say it's a nightmare but it's a huge disappointment."
As for the game against Juventus, he added: "I think we showed much better spirit than in the last few games.
"We did well and created some chances but in such an important game if you create chances you have to take them and that was the difference.
"They went ahead with a lucky deflection and the second goal was a deflection as well but they put on a very good performance and they did really well."