You can become a solid defender if you aren't the greatest athlete. Defense is 70% awareness, knowing the tendencies of the player your guarding, reading what the offensive team is trying to do, all stuff that is tied to basketball IQ. Doncic also has a big frame to add strength to, I don't think he will be bullied by wings once he fills out. He's probably gonna struggle laterally, which will come into play if he gets switched onto guards but you're drafting him for the hope his style of play is the kind that can change the culture of a team. Tim Duncan, Marc Gasol, Shane Battier were all great defenders but not great athletes, they just understand the game at a high level. Heck Draymond Green isn't a great athlete and he is probably the best defensive player in the league right now just because he reads the game and gives effort and an astounding level.