Doesn't matter if he was a professional in Europe. You need to go through the draft first. If he went undrafted, he could have signed as a free agent. While it was his choice to participate in the draft at age 19, he would still have been automatically eligible when he's 22 years old, like all players.
On a side note, the Doncic pick is looking pretty good right now. Marvin Bagley just went 3-16 from the field in his second game, it looks like he can't create for himself AT ALL, even against the summer league competition. Trae Young went 4/20 in the first game, 5/16 in the second game. One of the most horrific shot-selections ever. He has the mindset of Steph Curry, shooting range of Rajon Rondo. Bad defender too.
