Red Bulls defender Sean Niels collided violently with New York City FC defender Justin Haak in the 59th minute of Saturday's Eastern Conference semifinal, causing him to fall to the ground for several minutes.
The terrifying moment happened when the two players collided with each other for the ball inside the Red Bulls' box, with Nealis getting the worst of it.
Nealis appeared to be hit in the head with an elbow, hitting his head on the ground as he fell.
Citi Field fell silent as players from both teams watched with anxious expressions.
Haak was fine after a while, but the training staff thoroughly examined the Red Bull defender.
At one point, a stretcher was carried out for Nealis, but he was eventually able to stand up on his own and leave the field.
The delay lasted several minutes before play resumed.
The Red Bulls later announced that Niels suffered a concussion.
Noah Eir entered the game in the 62nd minute, replacing Niels.
At the time of the injury, the Red Bulls were 2-0 with two goals in the first half.
Felipe Carballo scored in the 16th minute, and Dante Vanzeil added another in the 25th minute.
Saturday's MLS Cup playoff game marked the first time the Red Bulls and NYCFC met in the postseason.
