A 16-year-old Bronx teenager says he was left with staples in his head after stepping in to stop a man from attacking a woman — and the teen says he's not sure he would intervene again.

Mostafa Tunkara and three friends chased the man after seeing him attack a woman near East 187th Street and Webster Avenue last Tuesday night, according to Tunkara and court records. Prosecutors say the suspect, 28-year-old Jon Arce, slashed one teen across the face and stabbed Tunkara in the head with a scalpel before fleeing.

Arce pleaded not guilty Thursday in Bronx Criminal Court to first-degree assault, attempted first-degree assault, second-degree assault, attempted second-degree assault, third-degree assault, criminal possession of a weapon and harassment, according to court records. A judge set bail at $100,000 cash or $250,000 bond.

He is being held at Rikers awaiting trial. His lawyer did not return phone messages.

Police never identified the woman they say was attacked.

"I just wanted to help her," Tunkara said. "If he had touched my neck or anything, I probably would have died."

Tunkara, born in the Bronx to immigrants from Gambia, said he was walking home from Fordham with three friends when he saw the man hit the woman, knock her to the pavement, kick her in the face and break her phone. The man told the teens the woman was his ex-girlfriend, then ran, Tunkara said.

“We just started chasing him and we almost caught him,” Tunkara said. “My other friend tried to punch him but he missed and that’s when he sliced my other friend's face.”

Tunkara said he tripped over his friend and was stabbed near the hairline as he stood up. He said he never saw the small scalpel before he was stabbed, an experience that left him reluctant to intervene again.

"I didn't see the weapon before he stabbed me, that's why," he said.

Tunkara required multiple staples to the top of his head. The other teen needed 15 stitches to close the wound, according to the complaint. Both were taken to St. Barnabas Hospital, treated and released, authorities said.

Assaults in the NYPD’s 48th Precinct, which includes Fordham Heights, are down from the past two years but well above pre-pandemic levels. So far this year, police have recorded 300 felony assaults in the precinct, compared with an average of 200 over the same period in the years before the pandemic.

Tunkara's father, Muhamand Tunkara, who does housekeeping at Montefiore Medical Center, returned home from a mosque to a call that his son was in the hospital. He said he is still troubled by his son's decision to step in for a stranger.

"I asked, 'Why are you fighting like that? Why did you do that?' I tell him, 'After six o'clock or seven o'clock, don't leave home. Stay home in the summertime,'" Muhamand Tunkara said.