Lewis, who scored the all-important equaliser in the Play-off semi-final first leg against Preston North End last Friday, must spend five days at a USA training camp in Cleveland, Ohio.
The Californian flyer has been given special dispensation by FIFA to take part in the Millennium Stadium showdown with Watford on May 21 – but the condition was that he attends a pre-World Cup training camp across the pond.
And, just 10 days ahead of the final, Lewis finds himself torn between club and country as he looks to prepare for two big occasions. A FIFA ruling states all players set to take part in the World Cup must embark on a rest period from May 14 – but the governing body have given Lewis the green light to return to Leeds for the play-off final.
American boss Bruce Arena also supported Leeds in their bid to secure the release of Lewis from international duty, but the former Preston winger won't be back with his United team-mates until the middle of next week.
By the time Lewis returns – just four days before the play-off final – he will have undergone a 7,240-mile round trip and spent over 20 hours travelling.
United boss Kevin Blackwell confirmed: "Eddie has had to go to an American training camp. He went yesterday and will be back on May 16.
"It was part of the agreement with FIFA to secure his release so it's just one of those things.
"We've just got to get on with it, and hopefully Eddie will come back okay.
"We're grateful to FIFA and Bruce Arena for Eddie's release and playing for his country in a World Cup is a big, big honour for him."
Lewis was the only Leeds player to emerge from Monday's game at Preston feeling any ill-effects.
Lewis was troubled by a minor thigh injury ahead of the game, but was struck down with illness during the clash.
"We were a little concerned that Eddie has picked up the virus that has done the rounds," said Blackwell.
"He was feeling unwell during the game and was the same before he went to America.
"He should be in good hands over there, though."
Lewis, who celebrates his 32nd birthday next week, will link up with the USA squad again immediately after United's play-off final clash with Watford.
"You want to extend your season for games like this," said Lewis.
"I've got nothing to complain about.
"I'll probably look back in a few years and wish I'd had a few more like this.
"I'm very settled in that regards."
Meanwhile, Blackwell says skipper Paul Butler is making good progress in his bid to shake off a calf injury.
Injury
Butler tore his calf during a training session ahead of the play-off semi-finals and is touch and go for the final.
Blackwell said: "Slowly but surely Butts is coming along."
Blackwell will face a major selection dilemma if his captain recovers in time for the final. Sean Gregan and Matthew Kilgallon were outstanding in the semi-finals, but Blackwell must shuffle his defence with Stephen Crainey set to miss out through suspension.

Tottenham chairman Daniel Levy has urged supporters not to let the club's failure to land a Champions League spot spoil an otherwise promising season.
Spurs slipped out of the all-important fourth spot in the Premiership when losing to West Ham on the final day of the campaign while arch-rivals Arsenal were beating Wigan.
And yesterday the Premier League quashed any hopes of having that match replayed due to the fact that 10 Tottenham players were hit by a bout of food poisoning on the day of the game.
Levy was naturally disappointed with that decision but said: "I have received many e-mails of support from you, our fans, welcoming our stand on this lack of fair play and I thank you for those.
"A recurring theme is the pride you all felt in our 'heroes' on the pitch on Sunday and that you are looking forward to our return to Europe.
"It has been a great season and we should refuse to let this mar the achievements."
However, Levy did also hit out at the appeal decision, saying he despaired at the state of the rules which did not take into account such matters as food poisoning.