Dan Koppen tears knee ligament

The Denver Broncos have confirmed that Dan Koppen will not be able to play during the 2013 NFL season after sustaining a knee ligament injury.

The centre, who rejoined the AFC West champions last month following JD Walton's ankle surgery, will undergo an operation to repair his snapped anterior cruciate ligament.

Koppen played 12 games for the Broncos last season after Walton suffered an ankle dislocation early in the campaign.

The team has signed former Seattle Seahawks centre Steve Vallos, pending a physical.