A judge released Stephenson, 19, without bail late Sunday night after his arrest on assault, menacing and harassment charges. Prosecutors had asked for $7,500 bail.
In a lengthy statement issued Monday, Pacers president Larry Bird called Stephenson's arrest "very disappointing to the Pacers franchise and to me personally." A defense attorney said Stephenson didn't harm his girlfriend.
Authorities say the confrontation occurred around 5 a.m. Sunday when the woman, Jasmine Williams, was returning home to her apartment building with two friends. Witnesses said Stephenson was waiting for her and yelled, "Are you kidding me?" before pushing her down the stairs.
A criminal complaint provided Monday by the Brooklyn district attorney's office said that as Williams lay at the bottom of the stairs, Stephenson picked up her head and slammed it on the bottom step.
When police arrived, Stephenson told them the woman had fallen down the stairs, prosecutors said. The complaint said Williams suffered head and back wounds and possible damage to her neck.
Stephenson's New York-based attorney, Alberto Ebanks, said Monday his client "did not maliciously, intentionally or in any other way cause harm to Jasmine Williams."
Ebanks, calling the charges against his client "very serious," added: "Everyone, especially Lance, regrets that this incident occurred. We look forward to addressing these charges in the appropriate venues."
Stephenson was one of the nation's top high school prospects. He played one season at Cincinnati before applying for early entry to the NBA draft.
Bird said the organization was still waiting to hear all the facts.
Still, Stephenson "should not have put himself in the position he was in early Sunday morning," the former Boston Celtics great added. "We have consistently emphasized to our players the importance of not putting themselves in situations where bad things can happen."
Depending on the investigation's outcome, Bird said, "we will deal appropriately with Lance so that he, the team and the entire Pacers community understands that this message cannot be ignored."