Brendan Aaronson reportedly had his appendix taken out in hospital in the week - yet still made the Leeds substitute bench for Sunday's defeat by Nottingham Forest.
The 22-year-old had an operation at the beginning of the week and left hospital on Wednesday, according to Major League Soccer reporter Tom Bogert.
Aaronson made the bench but was left unused by coach Jesse Marsch in a disappointing 1-0 defeat by Forest in the Premier League on Sunday.
Aaronson's condition meant that when his USA teammate Weston McKennie completed his loan move to Leeds from Juventus this week, he was unable to welcome him to Elland Road.
Tyler Adams took on the honor of greeting McKennie by himself - creeping up on the midfielder and surprising him in the immediate aftermath of putting pen to paper on his Leeds contract.
Adams, the USA captain, also held the first interview with McKennie on what Leeds fans can expect from the new signing.
McKennie was a substitute on Sunday, taking a place on the bench next to Aaronson and sitting under a blanket to stay warm in the English winter.
He came on in the 57th minute as Leeds chased an equalizer against Forest but his first game ended in a defeat.
It leaves Leeds level on points with 18th place Everton, which occupies the last place in the relegation zone despite beating league leader Arsenal 1-0 on Saturday.
Next for Leeds is fierce rival Manchester United in its next two Premier League games.
The first match, on Wednesday night, is at Old Trafford before Leeds hosts the following game on February 12.
Leeds coach Marsch hinted that McKennie could start in the week but Aaronson's status is likely to depend on the rest of his recovery.
'Weston has fitted in really well and had a really good week of training,' Marsch said.
'We have three matches this week, so we'll have to have some rotation, and certainly I think he'll feature from the start in one of these matches, but today from the bench I know he'll be ready.'
Aaronson joined Leeds from Red Bull Salzburg from $30m in the summer, along with Adams.
When all three are healthy, it means American coach Marsch could field a midfield with all three USA internationals.