Jürgen Klopp was criticised by fans of his former club Mainz on Saturday over his decision to join energy drinks company Red Bull and oversee its football clubs.

Mainz supporters held banners during their team's Bundesliga game against RB Leipzig -- a Red Bull-backed club -- in which they addressed their former hero.

"Have you forgotten everything we gave you?" asked a huge banner behind one of the goals, referring to his tearful farewell speech when he left the club after 18 years as a player and coach in 2008.

"Are you crazy?" asked another banner with a pun on Klopp's name using the German word "bekloppt" for crazy, while another referred to a quote of his: "I like people until they disappoint me."

Earlier this month, Red Bull announced that Klopp will become its head of global football from January, overseeing its international network of clubs, including Leipzig, Austrian team RB Salzburg, New York Red Bulls, Bragantino in Brazil and Omiya Ardija in Japan.

Red Bull also has a minority stake in English Championship club Leeds United, while it is set to become a minority stakeholder in French second-division club Paris FC.

After leaving Mainz in 2008, Klopp enjoyed successful coaching spells at Borussia Dortmund, then Liverpool, where he lifted the Premier League and Champions League.