Plymouth Albion centre Keni Fisilau has been given an 18-week ban for a dangerous tackle in their Championship game at London Scottish on 7 January.
The Tonga international pleaded guilty to the charge at a Rugby Football Union disciplinary hearing on Monday.
He will miss the rest of Plymouth's season and is not eligible to play again until 22 May.
Fisilau, who is the record Albion appearance holder, has been ordered to pay £250 towards the hearing costs.
The incident also involved Scottish's 19-year-old centre Mitchell Tamiau, who is dual registered with Premiership club Wasps.
The teenager broke his jaw in the tackle when, as he tried to retrieve a high pass, Fisilau caught him in the face with his head.
Albion are shocked at the severity of the punishment and are awaiting Fisilau's decision over whether he wants to appeal against the verdict.