Hamilton’s first Ferrari win in Barcelona closed the gap to 41 points behind Mercedes’ championship leader Kimi Antonelli.
Clark is bidding to become the first wire-to-wire US Open winner since Martin Kaymer in 2014.
The wing chose to risk a refracture in order to play in Saturday’s showpiece, which ended in a 26-17 defeat by Northampton.
An ECB statement said Gus Atkinson was ‘the victim of unprovoked attacks’ with Stokes ‘not involved in the altercation’.
The National Hunt meeting will now begin at 10.30am.