A 72-year-old cancer doctor was randomly slugged by a stranger in an unprovoked Upper East Side attack — telling The Post on Friday how his assailant coolly stood over him after the knock-down punch. 

“He didn’t say a word to me,” Dr. Jason Koutcher said of his still in-the-wind attacker, whose photo was released by cops this week. “He was walking the opposite way. And he walked just a step past me, and he gave me a huge punch.

“He looked at me as I was down.”

Koutcher, an oncologist at Memorial Sloan Kettering, said he’d been walking on Third Avenue between East 81st and East 82nd streets — on his way to synagogue, then work — the morning of Nov. 28 when he was suddenly assaulted, leaving him with a broken sternum.

“I remember I was thinking about needing to walk faster, because there was an article that just came out before that [said that] is better exercise and healthier,” Koutcher said. “And then he just came out of nowhere and hit me.

“When he hit me, I dropped,” the doc said, adding he made a grab for nearby scaffolding to try to steady himself.

“I had trouble catching my breath, which worried me, but then I sort of caught it,” Koutcher said. “I had a lot of pain as I went down.” 

The doctor noted the man didn’t rob him or try to take the backpack he was wearing. 

Koutcher called on City Hall to address homelessness in the city, which he blames for the attack, saying his assailant should “be in hospital for treatment or something.

“This is a problem in the city,” the doctor said. “I think that the mayor has a reasonable idea that he’s got to do something with the homeless.”

Mayor Eric Adams recently dramatically expanded the city’s ability to involuntarily commit New Yorkers with chronic and untreated mental illness, after a string of random horrifying attacks. 

Koutcher said he didn’t seek medical attention after his assault — until his wife urged him to.

“Doctors are not very good patients,” he admitted.

An X-ray showed that the punch had broken his sternum, Koutcher said. 

Koutcher added that now, he mostly takes a car to work.

Still, the septuagenarian scientist said he feels the East Side is safe.

“I’m not fearful of walking on the East Side. I think that this was a fluke,” he said.