The Canterbury Bulldogs have been handed a $20,000 fine from the NRL after Lachlan Lewis copped a knock on the head and failed to remove him from the field.

Lewis received a knock to the head attempting to tackle South Sydney's Josh Mansour during last week's contest.

The Bulldogs' playmaker went to ground and was having a hard time getting back to his feet.

Protocols about concussion are very strict about players copping a knock to the head, but Lewis remained on the field for two minutes before being taken off for assessment.

After being assessed, Lewis did not return to the field.

If the Bulldogs cannot explain why he wasn't immediately removed from the field, the could be slapped with a $20,000 fine.

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A statement issued by the league on Thursday said that the rules regarding concussion was clear and concise and the Bulldogs needed to explain themselves.

"Under NRL rules, any player who exhibits clear signs of motor in-coordination or possible motor in-coordination must be removed from the field immediately," the statement said.

“The NRL acknowledges the Bulldogs did view surveillance screen replays and removed Lewis from within two minutes of further elapsed game time.

“When considering the proposed penalty, the NRL has taken into account the club’s compliance to this part of the policy and that Lewis did not remain on the field for any tactical purpose.

“The NRL reminds clubs that all officials and coaching staff, beyond club doctors and trainers, have a responsibility to be alert to player reactions after head knocks during a game.”

The Bulldogs must respond to the league within five business days.