By Jamie Spencer Manchester United welcome Leny Yoro to the club this summer, hoping the Frenchman will be the kind of smash hit Real Madrid achieved with an 18-year-old Raphael Varane. The £52m deal highlights just how convinced United are that he is destined for greatness, even while still a teenager with only 60 first-team appearances under his belt. Many of United’s very best signings over the years are those who have joined the club as burgeoning talents not yet at their peak – think Cristiano Ronaldo, Wayne Rooney and David de Gea, even dating back to Roy Keane or Ole Gunnar Solskjaer. L…