In a statement released Sunday, the Nets said Harden was diagnosed with right hamstring tightness. The expectation is for the team to monitor how his hamstring responds to treatment over the next few days.
"This is playoff time. This is time to take some risks, but it has to be right to take those risks -- we're not going to take any old risk. So, we hope he's back. He believes he can be back. But, we've got to see what happens and only time can tell how he responds."
Harden appeared to suffer the hamstring injury when he attempted to lob a pass over Bucks guard Jrue Holiday and center Brook Lopez early in the first quarter of Game 1. But after he lobbed the pass, Harden landed and immediately grabbed at his right hamstring.
The 31-year-old Harden eventually left the floor and retreated to the Nets' locker room.
During the regular season, Harden missed more than a month because of a right hamstring strain.
The Nets, who hold a 1-0 series lead over the Bucks, also will be without veteran forward Jeff Green in Game 2. Green has been sidelined for over a week due to a plantar fascia strain in his left foot.
"I think we'll see him in this series," said Nash, who noted that Green has been progressing. "And hopefully, it's Game 3. But who knows."
Game 2 of the best-of-seven series is scheduled for Monday night at Barclays Center in Brooklyn.