The rim-rocking rejection came in the first quarter of the Nets' 115-110 triumph on Tuesday at Barclay's Center in Brooklyn. Lonzo Ball dribbled down the left flank at the start of the sequence. The Lakers guard passed to a trailing James at the top of the 3-point line.
James ran past Rondae Hollis-Jefferson before elevating through the paint. Allen crouched under the rim and waited for James before jumping straight up. He used his right hand to deny James of the hoop, spinning the ball away from James' right palm as he slammed his arm down.
"He's a shot-blocker," James told reporters. "Sure, he can get a block. It happens. It's probably all over social media. It's cool."
James still scored a game-high 36 points and had 13 rebounds and eight assists in the loss. Allen scored six points and had eight rebounds, two blocks and two steals for the Nets. D'Angelo Russell paced Brooklyn with 22 points and 13 assists.
The Lakers host the New Orleans Pelicans at 10:30 p.m. on Friday at Staples Center in Los Angeles.