United have appealed to FIFA for permission to sign Forde after his transfer was blocked by the Scottish Football Association on deadline day.
Manager Craig Levein, who has named Lee Wilkie as United's new club captain following Barry Robson's move to Celtic, had agreed a loan with the Bluebirds for 28-year-old Forde.
It would have allowed him to send Scotland Under-21 goalkeeper Euan McLean out on loan before a decision is made on his long-term future.
However Forde fell foul of the ruling which prevents a player turning out for three clubs within the same season when it emerged he had played for Cardiff in a Welsh Cup tie against tiny Welshpool Town last month, in front of just 700 fans.
Levein said: "I'm really annoyed about this.
"I tried to sign David on Thursday night but it was blocked because of this rule about playing for three teams in a season.
"He went on an emergency loan from Cardiff to Luton but because they are in the same league structure English clubs have an agreement that it doesn't count as a transfer.
"Because he had already played for Cardiff in a Welsh Cup tie it has been blocked and our friends at the SFA have refused to recognise the English agreement.
"I've taken it to FIFA to give an answer on it.
"The paperwork has been done before the window closed so if we get the go-ahead it can still go through."