West Ham technical director Tim Steidten has launched a move for a star who played for Celtic U20s aged just 13. West Ham are embarking on a major rebuild this summer after the appointment of Julen Lopetegui. And the Hammers are doing it around ‘young, exciting’ stars. That came from the horse’s mouth when technical director Tim Steidten unveiled Brazilian teenager Luis Guilherme as West Ham’s first signing of the summer last week. The 18-year-old signed for West Ham from Palmeiras for £25m. And Steidten revealed he has promised Guilherme he will be joined by a host of other up-and-coming star…