Irving has been ruled out of the game at Barclays Center after an MRI confirmed that he suffered a right ankle sprain in Sunday's Game 4. Harden, who strained his hamstring in Game 1 of the second-round series, also remains out.
Nets head coach Steve Nash told reporters he has "no idea" whether Irving will return in the best-of-seven series against the Bucks, which is tied at two games apiece after two straight Milwaukee wins.
"I have to wait and see how it progresses, I think," Nash said Monday. "And then the experts would have to wait and see how it progresses and give you a clearer picture on what the window would be for his return."
Irving sustained the ankle injury halfway through the second quarter of Sunday's contest when he landed awkwardly on Bucks star Giannis Antetokounmpo's foot after sinking 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 the Nets' Game 4 loss.
Nash said that Harden, who has been out since June 5, has done some on-court work since his injury, but he still needed to complete multiple high intensity workouts without any setbacks before being cleared to compete.
Nets star forward Kevin Durant has played in five games this season without both Harden and Irving. In those matchups, he averaged nearly 34 points per game and shot an efficient 63% from the floor.
Game 5 of the series is set for Tuesday night at 8:30 p.m. EDT in Brooklyn.