Why has Ralf Rangnick failed at Manchester United? 

Ralf Rangnick is a German professional football manager, executive and former player who is currently the interim manager of Premier League club Manchester United. After an uneventful career as a player, Rangnick began his coaching career in 1983, at age 25. 

Manchester United Football Club is a professional football club based in Old Trafford, Greater Manchester, England, that competes in the Premier League, the top flight of English football.

Steffen Prößdorf, CC BY-SA 4.0 , via Wikimedia Commons