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 18-year-old was riding Virtue Chastity for Amanda Perrett.