Celtic confirmed the appointment of the 55-year-old Australian as their new manager on Thursday following a long and arduous search for Neil Lennon’s successor at Parkhead.
Postecoglou is relatively unknown in European football but he is a big name in Australian football and also won a league title in Japan at Yokohama F. Marinos.
Following their public failure to land Eddie Howe, the Celtic board’s decision to bring in Postecoglou has underwhelmed many of the club’s supporters.
But Broich, who played under the Australian at Brisbane Roar, stressed that his former manager has it in him to succeed in Europe.
The German conceded that the pressure at European clubs is different but insisted that despite coming through in Germany, Postecoglou remains the best manager he worked with in his career.
Broich told The Athletic: “I believe in him, looking at his successes, it’s based on a deep, deep understanding of the game, and of the psyche of players.
“We all know that sometimes it can be difficult in European environments, in terms of pressure and expectation, so there is no guarantee of success.
“But I can tell you I have worked in German football for long time as a player, as a coach, and as an expert.
“To me personally, to this day, he is the best manager I have ever encountered in football.”
Postecoglou has a tough task on his hands in reviving a Celtic team that finished 25 points behind the champions Rangers last season.