Chelsea had played a 1-1 draw with Real Madrid in the first-leg last week in Spain.
But Cascarino feels Timo Werner needs to take his chances for Chelsea against Real Madrid on Wednesday night, adding that Tuchel must find a way to stop Karim Benzema in order to qualify for the Champions League final ahead of the Los Blancos.
“Thomas Tuchel has been brilliant at getting his team to take an iron grip on matches, keeping the opposition out of games after games, but he needs his attacking players to make the most of their chances,” Cascarino told The Times.
“Timo Werner has been producing some wonderful passes and has been providing great movement and creativity, but he has to be more clinical in front of goal.
“If Chelsea gets knocked out tonight, and Werner has missed one or two good chances, then the finger will be pointed at him.”
He added, “Chelsea must keep a very close eye on Karim Benzema, who’s probably the most underrated centre forward to have played in my lifetime.”