The 41-year-old cast doubt over his participation in the one-lap mini race ahead of the Silverstone showpiece.
Clark called last year’s incident at Oakmont the “lowest point” of his career.
Van Poortvliet has edged Alex Mitchell.
The Lancashire seamer impressed in Wednesday’s Vitality series opener against India, which was washed out.
Retired riding great ‘very sore’ in hospital after incident in Newmarket on Wednesday evening.