The game had to be stopped twice late on, the first time after Hertha fans threw fireworks on to the pitch and the second when Fortuna supporters invaded it with one minute remaining, prompting a 15-minute delay as the teams to left the pitch and police and stewards tried to restore order.
The match ended 2-2 on the night, with Fortuna winning 4-3 on aggregate to replace Hertha in the Bundesliga next season.
The pitch invasion came during seven minutes of stoppage time and Rehhagel was disturbed by the scenes.
"It was catastrophic, I never experienced anything like it," said the 73-year-old.
"I could not have imagined something like that being possible in Dusseldorf.
"It really could have ended in a catastrophe."
Fortuna coach Norbert Meier praised the way in which referee Wolfgang Stark dealt with the situation.
"We all want to see football," he said.
"Quite honestly, I was pleased that Mr Stark kept such a cool head and was a relaxing influence on both sides.
"Compliments must go to Mr Stark for his performance."
Hertha finished the game with 10 men and Rehhagel claimed Anis Ben-Hatira's red card cost the capital club their place in the Bundesliga.
Ben-Hatira was sent off in the 54th minute of the match, picking up a second yellow card when the game was finely poised at 1-1 and Rehhagel's side were pushing for a goal which would have levelled the tie.
"Of course it is a great disappointment for us that we were unfortunately unable to make it," said Rehhagel, whose short-term contract has now expired.
"We had to play with a man less yet again. If we had remained with 11 men then we would have been able to cause them more problems.
"We were only a few inches away and had we not got the red card, then I think we would have been able to gain more from this game."
But he added: "We are allowed to cry for one night, but then we have to look forward."
Meier was full of praise for his side as they sealed their return to the Bundesliga after a 15-year absence.
"I can only praise my team as this is a small sensation," he said.
"For a city like Dusseldorf, which is a football city, it is fantastic."