By Matthew Cooper Manchester United legend Roy Keane has opened up about the infamous moment Sir Alex Ferguson kicked a boot at David Beckham following a game against Arsenal. The boot accidentally hit Beckham in the face and left him with a cut above his left eye. And Keane, who captained United as they were beaten 2-0 in the FA Cup fifth round, has revealed why the incident occurred. Speaking inside the dressing room at Old Trafford, Keane told Sky Sports: “I think Becks was over there somewhere, somewhere there at the mirror. I think the manager had a go at him for not tracking back and I t…