The Dallas Mavericks did the unthinkable overnight by trading Luka Dončić.It's perhaps the most stunning trade in NBA, and maybe sports, history – there had been no rumors about this at all, and Dončić is still just 25 years old and a perennial MVP candidate.SIGN UP FOR TUBI AND STREAM SUPER BOWL LIX FOR FREESo, why would the Mavs move on from him? Especially after an NBA Finals appearance.CLICK HERE FOR MORE SPORTS COVERAGE ON FOXNEWS.COMWell, general manager Nico Harrison admitted shortly after making the trade that he was concerned about the team's defense, a bugaboo of Dončić's. Trading for