By Matthew Cooper Former Manchester United manager Ralf Rangnick and Borussia Dortmund chief Hans-Joachim Watzke have defended Jadon Sancho, insisting the winger does not have a “problem with discipline”. Sancho has rejoined Dortmund on loan this month, less than three years after he left the club to join United in a bumper £73million deal. The 23-year-old struggled to make much of an impact at Old Trafford and had been exiled from the first team by manager Erik ten Hag following a public falling out in September. After Sancho was left out of United’s squad for their 3-1 defeat to Arsenal, Ten…