The 30-year-old played a full 90 minutes for AZ reserves Monday night against Ajax wearing a helmet understood to be a heavier version of the one used by Chelsea keeper Petr Cech.
Didulica was injured on October 22, 2006 playing in AZ's home game against PSV Eindhoven.
He was hit at close-range by a shot from compatriot Jason Culina and went on to suffer headaches and dizziness for a year.