The Mavs announced Wednesday that Nelson and the team "mutually agreed to part ways," but sources told ESPN that owner Mark Cuban fired Nelson on Sunday.
"It was kind of tough to me," Doncic said during a Thursday media availability for the Slovenian national team. "I really like Donnie. I know him since I was a kid and he was the one that drafted me. It was tough for me seeing that, but I'm not the one making decisions there."
Doncic's comments came before coach Rick Carlisle told ESPN's Adrian Wojnarowski that he would not be returning to the Mavericks next season. Carlisle had two years left on his contract. He spent 13 seasons as Mavericks coach.
Nelson earned a reputation as an international scouting pioneer during his 36 years in the NBA, including the last 24 years with the Mavs, who played in two NBA Finals and won one title under him. His two most impactful acquisitions were the draft-day trades for Dirk Nowitzki in 1998 and Doncic in 2018.
Doncic, 22, has been a first-team All-NBA selection the last two seasons. As a result, he is eligible to sign a supermax extension this summer, a five-year deal worth an estimated $201 million. Sources told ESPN that Doncic would likely get a player option for the fifth season, as the Utah Jazz's Donovan Mitchell and Boston Celtics' Jayson Tatum did on their rookie extensions last summer.
A day after the Mavs' season ended with a Game 7 loss to the LA Clippers, Doncic strongly hinted he would sign the extension.
"I think you know the answer," Doncic said, with a big smile.