The 73-year-old coach has seen his side pick up just eight points in 11 games since replacing Michael Skibbe in charge and he has acknowledged that the chances of his club remaining in the top flight are looking slim.
"We have got to win on Saturday and Cologne have got to lose, but I don't know if we will win," he said.
"We have not been playing at home the way I would have imagined it.
"But I think we have a small chance and we have got to take it. We have got to work on winning this game with the support of many fans."
Apart from a 1-0 win over Werder Bremen, Rehhagel has lost his other four home matches and must now reverse that trend against a side coached by one of his predecessors Markus Babbel.
Babbel was forced out of the club in December after declining their offer of a new contract, and his controversial departure will make him even more desperate to win next weekend.
"I am 100% certain that Hertha would not have gone down if I were still coach," said Babbel to the Tagesspiegel am Sonntag newspaper.
"Everything was working excellently between the team and the coaching team."
A difference of opinion between Babbel and the club's director of sport Michael Preetz was the only problem, resulting in his premature departure, and Berlin have won just twice in 17 games since.