Preetz said the 73-year-old Rehhagel, who won German and European titles as coach of Werder Bremen and Kaiserslautern in the 1980s and 1990s before leading lowly Greece to the European title, was the right man to take the club out of their crisis.
"We are currently in a difficult situation and reached the conclusion that we need an experienced coach like Rehhagel to reach our goal of [staying up] at the end of the season," Preetz told reporters.
"He has agreed to help us until the end of the season."
Rehhagel, who will be the season's oldest Bundesliga coach once he takes over, would arrive in Berlin on Sunday, Preetz said.
Hertha, who are trying to avoid the drop again after winning promotion last year, sacked Michael Skibbe last week after just five games in charge. Hertha lost all five games. He had replaced Markus Babbel in December.
The youthful Rehhagel, who also had a spell at Bayern Munich in 1995, last coached in the Bundesliga in 2000 before leaving Kaiserslautern to take over Greece from 2001 until 2010 and stunning the football world with their surprise Euro 2004 victory that earned him the nickname 'King Otto'.
He won two German league titles with Bremen as well as the now-defunct Cup Winners' Cup while also becoming the only coach to lead a newly-promoted team straight to the Bundesliga title when he did it with Kaiserslautern in 1998.
A former Hertha player himself, he also played in the inaugural Bundesliga season in 1963.
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