You never want to see a player go down, and you definitely don't want to see a player go down and stay motionless for a while after hitting his head.  That was the scary moment the Celtics went through when Mickael Pietrus took a hard fall on Friday night. 

The Boston Celtics suffered a 99-86 loss to first place Philadelphia on Friday night, but that could have been made far worse when forward Mickael Pietrus went down hard on his head.

In the video you can see Pietrus fall to the floor, and immediately grab for the back of his head.  he did lay on the ground motionless for a while before being taken off in a stretcher.  They initially believed  it could be a closed head injury, and he had several tests done, but did not stay in the hospital, according to Boston.com.