"We've told the potential investors we need to know their level of interest by the end of March," Tatum told a group of journalists in Los Angeles ahead of the NBA's All-Star weekend.
"That's the timeline where we're expecting these dozens of investors to come back and say, 'Yes, we're interested in this market, at this price,'" Tatum added.
The league aims to finalize the project by autumn 2027, Tatum said.
The proposed competition would feature 12 permanent franchises and four qualifying teams, created in partnership with FIBA, basketball's global governing body.
Target cities include Paris, Lyon, London, Berlin, Madrid, Barcelona, Milan, Rome, Manchester, Munich, Athens, and Istanbul.
Major football clubs such as Paris Saint-Germain, Inter Milan, and AC Milan have been approached to form franchises for the league.
The plan has sparked tension with the EuroLeague, Europe's current top competition, which recently named Spaniard Chus Bueno as president.
Tatum, though, is optimistic that Bueno, a former NBA executive, will help the league come to fruition.
"We do know Chus Bueno, he obviously worked for us for a dozen years or so," Tatum said.
"We've always said from the very beginning we've wanted to work together with the EuroLeague to align everyone and get everyone from the ecosystem aligned.
"I think with having Chus there, somebody who knows us well, who knows European basketball well, perhaps he can be a bridge between us, FIBA, and the EuroLeague."