A little-known FA law states that clubs can agree between themselves to play 30 minutes of extra time at the end of the original tie in a bid to avoid having to play the match again.
With United manager Alex Ferguson facing a crippling injury crisis as his side chase silverware at home and in Europe the last thing he needs is a replay in London squeezed into an already packed schedule, especially with the Champions League about to resume.
Likewise, Tottenham manager Harry Redknapp, whose side will face United in the League Cup final, has vowed to field a "mish-mash" side for a match he described as secondary to their relegation six-pointer against Stoke City next week.
A quick glance at FA Cup Rule 12b may have spared both men the inconvenience of an unwanted extra fixture but Ferguson appeared unaware on Friday when he read out a long list of players unavailable for Saturday's tie.
The rule states that extra time can be played at the end of match if both clubs agree in writing within seven days of the draw being made and the referee is notified before kickoff.
In normal circumstances there is only extra time at the end of a replay with penalties to follow if it is still level.
"Someone has informed me that I could apply to The FA to have extra time tomorrow," Ferguson told reporters. "This has come out this morning, I've never heard of this in my life.
"It's in the articles of The Association that both teams can apply to have the game decided tomorrow and notify the referee before the game.
"I'm going to speak to David Gill and ask him first of all if this is actually correct. I've never heard of it. The criticism would be that we're not adhering to the principles of the FA Cup or the spirit of it -- but why have The FA got it in there?"
An FA spokesman said on Friday that the rule has not been invoked from the first round proper onwards for at least 50 years but is sometimes used in preliminary rounds to spare amateur clubs huge travel costs.
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