Video: 'He dropped like a cow shot in an abattoir' - watch why Sean O'Brien was subbed
Sean O'Brien had to be substituted after making a rampaging run in the first half of Ireland's Six nations encounter with England.
He fell backwards when he trued to regain his feet and the Irish doctor Eanna Falvey rushed onto the pitch and quickly helped the Carlow man from the field.
Speaking on RTE at half time, George Hook said: "Sean O'Brien dropped like a cow shot in an abattoir.
"It is very important to understand that you do not have to be hit on the head to suffer a brain injury.
"This is really worrying for the player and ultimately for the game."
View the incident here: