The Argentina midfielder was sent off in the 84th minute after he reacted angrily to a James McClean challenge.
Watford said his punishment was excessive and that he had been wrongly dismissed, but the latter was rejected by the Football Association.
Pereyra will now only miss Saturday's game against Everton.
Watford's Roberto Pereyra has had his suspension reduced from three matches to one following his sending off in Saturday's 3-1 defeat by West Brom.