After the Bucks evened the series at two, head coach Steve Nash told reporters X-rays on the injury were negative and Irving will undergo further testing.
The injury occurred midway through the second quarter with Irving hitting the floor after going up for a layup in the paint.
With the guard in obvious pain, players and fans around the league quickly took to social media to wish the Duke alum well.
Irving, 29, averaged 26.9 points on 50.6 percent shooting, 6.0 assists and 4.8 rebounds in 54 games for the Nets, who went 48-24 and finished second in the Eastern Conference during the 2020-21 regular season. He tallied 11 points, five rebounds and two assists in 17 minutes before getting helped back to the locker room.
Brooklyn entered Game 4 with a 2-1 series lead but will head back Barclays Center for Game 5 on Tuesday with the series tied.
The Nets enjoyed the benefits of its new big three featuring Irving, James Harden and Kevin Durant, but all three players missed long stretches of the season because of injury, COVID-19 protocols and/or personal reasons.
Still, the Nets were able to finish near the top of the East in part because of Irving, who was sensational this season and earned his seventh All-Star appearance.
Unfortunately, the Nets will be without Irving in addition to Harden who is currently out with a hamstring injury, which likely means that Bruce Brown will step into the lineup and play shooting guard. This could also mean more time for point guard Mike James off the bench if Irving is out going forward.