sentences:  Williamson sustained a shoulder injury in a fall at Kempton Park Racecourse ten days before the Epsom Derby and after a slow recovery such that he was not able to ride before Derby Day, he was replaced by Lester Piggott.Roberto started the 3/1 favourite for the Derby in a field of twenty-two runners.He was sent to the front by Piggott inside the two furlong pole but was strongly challenged all the way to the finishing line by the outsider Rheingold.