Goalkeeper Alan Mansfield will miss the biggest game in Horsham's history.
The former Three Bridges stopper had an appeal for a sending off turned down by the FA and will serve a one-match ban when the Hornets take on Swansea in next week's FA Cup second round clash at Queen Street.
Mansfield was shown a straight red card for deliberate handball in the late stages of last week's 5-1 Ryman League Cup win against Eastbourne Town.
Horsham were hoping that video evidence would help overturn the decision.
Club secretary John Lines said: "The FA say that under FIFA regulations they are not permitted to accept a downgrading request and so his suspension stands. There is also no mechanism for appeal now."
Recent signing Paul Seuke will take Mansfield's place against Swansea.
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