Cape Town - Ronald Koeman has urged Everton midfielder Ross Barkley to prove his worth after being left out of Sam Allardyce's first England squad.The Toffees playmaker started the season well and was expected to be a permanent fixture in the Three Lions squad after forging a reputation as one of the most creative and exciting young English talents in the top-flight.Barkley scored 12 goals last season and already has two in his opening four matches under new head coach Koeman, who now urged him to use the disappointment of being left out as a driving force to better performances on the field."Let's talk about Everton, because we are now behind, after the international break," the Dutch coach said."What he needs to do is to play good football, to show his qualities and show Big Sam every weekend that he needs to select the player. That is the best answer for the player by himself."The 22-year-old Barkley has scored two goals in 22 games for the senior national team since making his debut in 2013.