Doncic did not play in qualification for the 2019 World Cup and the 2017 European champions failed to make the tournament in China.
Seven nations secured Olympic berths at the World Cup, joining hosts Japan.
The four remaining places will be decided in a 24-team tournament scheduled for June 23-28 next year.
Two-thirds of the qualifying tournament slots had already been allocated to the highest-ranked sides not already qualified for Tokyo.
The final eight spots were allocated to the two highest-ranked nations in each of basketball’s four global regions when the governing body of world basketball (FIBA) announced its latest rankings on Thursday.
Slovenia and Croatia secured the two European spots. China and Korea took the two Asian places. They were joined by Angola and Senegal from Africa and Mexico and Uruguay from the Americas.
Olympics men’s basketball tournament (12 teams)
Already qualified (8): Japan (hosts), Spain, France, USA, Argentina, Nigeria, Australia, Iran
In final qualifying tournament (24): Angola, Brazil, Canada, China, Croatia, Czech Republic, Dominican Republic, Germany, Greece, Italy, Korea, Lithuania, Mexico, New Zealand, Poland, Puerto Rico, Russia, Senegal, Serbia, Slovenia, Tunisia, Turkey, Uruguay, Venezuela