Nets head coach Steve Nash said after the game that X-rays were negative, and Irving will undergo further testing and treatment. The team didn't provide a timetable for his return.
"We'll have to see how it goes," Nash said. "We'll cross our fingers."
Irving suffered the ankle injury with about six minutes remaining in the second quarter when he landed awkwardly on Bucks star Giannis Antetokounmpo's foot after making a layup. Irving remained on the floor for several minutes as Nash and team trainers attended to him.
The 29-year-old Irving then limped to the locker room without assistance and didn't return to the court. He finished with 11 points and five rebounds.
In Irving's absence, the Bucks pulled away from the Nets for a 107-96 win to even the best-of-seven series at 2-2.
The Nets already were without star guard James Harden, who has been out since June 5 because of a right hamstring strain.
Without Irving and Harden, the Nets relied on Kevin Durant, who played 42 minutes in the loss. He scored 28 points on 9-of-25 shooting from the field and recorded 13 rebounds.
Veteran forwards Blake Griffin and Joe Harris each scored eight points. Jeff Green, who had been out since the Nets' first-round series against the Boston Celtics, also added eight points in his return from a strained plantar fascia.
Antetokounmpo led the Bucks with 34 points and 12 rebounds. Khris Middleton contributed 19 points and eight assists, and Jrue Holiday added 14 points and nine assists.
"We're very happy, but we've got to keep getting better, keep playing together and hopefully we can go into Brooklyn and take one," Antetokounmpo said.
The Bucks erased a 2-0 deficit by winning two straight games at Fiserv Forum in Milwaukee. Game 5 is Tuesday night at Barclays Center in Brooklyn.