It's sad when a deliberate knee puts out a Star & there is no punishment.
I've always felt that soccer was the last sport in which opponents went after the Star;
sorry Basketball has a double penalty base for deliberate attempt to injure.
As a retired referee in soccer, swearing at a Ref. was considered the same as hitting a ref.= out of soccer for a year at least.
Deliberate injury of a player was a RED card. out of the game & subject to further penalties after a review.  It only happens to the stars of the game in a deliberate act- knee up is deliberate.