By Sean Walsh Barcelona head coach Xavi has conceded that his side have received the ‘most difficult opponent’ in their Europa League play-off after being drawn with Manchester United. For a second season running, Xavi has failed to guide Barça through the group stages of the Champions League and will return to Europe’s second-tier competition on the other side of Christmas. After already facing Bayern Munich and Inter on the continent this term, Barcelona’s life has not been made much easier after being paired with Man Utd in the play-off round – the Red Devils finished second to La Liga side…