Manchester United have had their fingers burned before. Time and again, the Red Devils would bring in established big names who had been allowed to leave a European giant before discovering exactly why they were allowed to leave in the first place. See the injury-prone Raphael Varane and the rapidly-declining Casemiro. Alexis Sanchez too. Devastating on his day but no longer the same irrepressible force of old. Ditto Radamel Falcao. Bastian Schweinsteiger’s spell at Old Trafford also proved highly disappointing. The man who pulled the strings during Germany’s 2014 World Cup triumph arriving at…