Should the Red Devils fail to pick up a point in the capital on Saturday, they will become the first side to go down to the second division this season.
Hertha could become the second next weekend, though, if they fail to pick up maximum points at the Olympiastadion.
"This is the most important game of the season and I hope our lads play this game like second half in Leverkusen," said Rehhagel.
"At the same time, although there is so much at stake, they need to remain level-headed."
Hertha have picked up just eight points in nine games since Rehhagel took charge.
As a result, they are now in a worse league position than they were when he replaced Michael Skibbe in February.
Although their situation is not as bad as Kaiserslautern's, the former Greece coach says victory cannot be taken for granted.
"I just have to warn that people should not think that, just because we are facing a side who are likely to go down, that it will be easy," he said at the pre-match press conference.
"They will not gift us anything. We need to be top fit, mentally and physically, to win this game."
Kaiserslautern have not won for over half a year, but they want to taste victory at least once before the end of the season, even if they are resigned to their destiny.
"We will keep fighting until we win again," said their coach Krassimir Balakov.
"I have tried to build the players up with optimism."
Christian Tiffert is suspended for the trip to Berlin while Julian Derstroff is ill.
Hertha's injury woes have eased with Christian Lell, Adrian Ramos, Andreas Ottl and Anis Ben-Hatira all now back in training.