"I am confident that we will play Bayern Munich but I take [the threat] very seriously," said Heusler.
Fifa's clash with Sion began in 2009 when the Swiss side received a one-year transfer ban for breaking transfer regulations in the signing of the goalkeeper Essam El-Hadary.
The club launched a series of civil court actions after being thrown out of the Europa League.
Yesterday Fifa complained about "the attitude of the club repeatedly trying to circumvent this decision in a legally abusive manner", in a statement issued in Tokyo.
Fifa and Uefa's hardline stance was last week backed by the Court of Arbitration for Sport. Fifa has now given the Swiss FA until January 13 to nullify all results involving Sion where they have fielded ineligible players.
Under the terms of a Fifa suspension, a country's national and club teams, referees and officials are prohibited from taking part in international matches and meetings, or receiving Fifa funding.
Meanwhile, United's deal for Gaitan will see the Argentinemidfielder move to Old Trafford next summer. While no firm contractual agreement has been reached between the two clubs, an arrangement is in place to bring Gaitan to Old Trafford in June.
United have refused to meet the £37.7m release clause figure written into the 23-year-old's new Benfica contract in October, but the sum the Premier League champions pay could rise to £33.5m if the 23-year-old has a prolific second half to the season.
The final transfer fee will be contingent on the player's appearances and performances, with Benfica's requirement to pay Boca Juniors 20 per cent of the final fee - probably £7 million - under the terms of the deal by which they signed him in May 2010.
Sources in Portugal suggested last night that United's own performances in domestic and European competition may also affect the final price they pay for a player who has netted only once this season and scored seven times in the last campaign.
Sir Alex Ferguson said that United, deprived of both Darren Fletcher and Anderson in midfield and with Tom Cleverley unlikely to return until well into January, would only buy in the winter window if "a player we have always been interested in suddenly appears", and Benfica are extremely reluctant to sell next month.
As well as progressing to the Champions League knockout stage, the club faces an intense battle for Portugal's Liga with Porto. Having wrested the title away from that club in 2010, they sold players and relinquished it back to them last season.
Their 5-1 win over Rio Ave on Friday night put them three points clear at the top, ahead of second-placed Porto's clash with Maritimo last night.
The Benfica board are likely to deny any suggestion of a deal with United for Gaitan, because intense pressure from fans not to sell players makes any public acknowledgment impossible, and Portuguese stock market rules dictate that the club must first formally disclose any sales or purchases.
- Independent
