By Martin Domin Boxing legend Lennox Lewis has told Anthony Joshua that defeat by Francis Ngannou is unthinkable. Joshua will take on former UFC heavyweight world champion Ngannou in Saudi Arabia on March 8, less than three months since he stopped Otto Wallin in the Middle East to reignite his career. Ngannou, meanwhile, almost shocked Tyson Fury last year when he dropped the boxing champion before losing a narrow points decision. Joshua is the odds-on favourite to stop Ngannou who will be taking part in only his second-ever professional boxing fight. And Lewis, the last undisputed heavyweight…