Howell, 29, was injured in a collision with Vikings prop Ben Cross, and was taken to hospital after the game.
A Broncos statement said: “No set time has been set for Howell’s recovery period at this stage. Everyone at the club wishes David a speedy recovery.”
The Australian scored three tries in 20 appearances for the club last season.
Meanwhile, the injury that forced scrum half Craig Gower to leave the field during the first half “doesn’t look too serious”, although the club have not revealed if could be fit to play in the round two game against Huddersfield.