Riesgo sustained the injury in his left foot during training yesterday and, according to Osasuna's official Twitter feed, the 29-year-old is set to be out for "between two to three months".
The former Spain U-21 international is the back-up for first choice keeper Andres Fernandez.
Osasuna currently lie in La Liga's relegation zone, two points above bottom club Deportivo La Coruna.
Source :FIFA