Fulham approached out-of-contract Ashdown with a view to using him against Leeds on Saturday after their first-choice keeper, Marcus Bettinelli, was sent off in a 5-0 defeat to Watford last Friday.
Bettinelli received an immediate suspension, leaving Gabor Kiraly as Fulham’s only experienced alternative to the 22-year-old Italian.
Ashdown is without a club after finishing a short spell at Crawley but Football League rules have prevented Fulham from making any new signings until the start of January.
The 34-year-old, who left Leeds on a free transfer last summer, said: “I’m really gutted not to have the chance to come up against Leeds and show my wares in the Championship again.
“Fulham would have been a fantastic opportunity to show what I’m capable of and playing against Leeds, where I never really had a look in, would have been the icing on the cake.”
Ashdown’s two-year spell at Elland Road was badly affected by a toe injury and he was routinely kept out of United’s starting line-up by Paddy Kenny.
The former Portsmouth keeper made eight competitive appearances in cup fixtures but did not feature in a single league game.