The incident, for which Portis was assessed a flagrant foul 2 and ejected, occurred with 8:02 remaining in the second quarter of the Knicks' 117-87 loss to the Los Angeles Lakers on Tuesday night at Staples Center.
The Lakers led the Knicks 39-30 with 8:02 remaining in the second quarter when Kentavious Caldwell-Pope streaked in for a transition layup off a Rajon Rondo feed. Portis, trailing from behind, swung his right arm and struck the top of Caldwell-Pope's head with the inside of his biceps.
The blow sent Caldwell-Pope's headband flying, and the play ended with Caldwell-Pope on the floor wincing in pain while several of his Lakers teammates -- LeBron James and Rondo among them -- confronted Portis.
Caldwell-Pope, after rubbing the sides of his head while being attended to by Lakers head athletic trainer Nina Hsieh as he stayed on the floor, subbed out of the game to receive further medical attention.
Lakers guard Danny Green was selected to shoot free throws in Caldwell-Pope's place, going 1-for-2.
The Lakers said Caldwell-Pope underwent a concussion evaluation, which was negative. He checked back in with 2:21 remaining in the third quarter.
"It wasn't intentional at all," Portis said in the postgame. "I went for the ball. My intent was never to hurt anybody or anything like that. Just trying to make a play on the ball. Missed the ball and hit him in the head. My apologies to Caldwell-Pope. Happy to see him go out and do his thing, come back into the game and impact the game."