James Constable will miss the next four Oxford United games after an appeal against his red card last weekend was rejected by the Football Association.
The striker was dismissed for violent conduct in the 3-0 win at AFC Wimbledon, but the U's felt video evidence proved his innocence.
After reviewing the footage, a disciplinary panel at the FA upheld referee Lee Collins's original decision.
It means the 28-year-old is now suspended for four games, starting with Saturday's FA Cup third round tie against Sheffield United at the Kassam Stadium.
He also misses the Johnstone's Paint Trophy clash with Southend United, plus npower League Two games against Bradford City and Rotherham United.