I can't stand when ppl say a I've got the flu (with intended meaning of 'stomach flu'), as your first thought is influenza.
'Stomach flu' or more accurately gastroenteritis, is 24 hrs of N/V/D.
'Flu' or influenza is 3 days to 2 weeks of fever/HA/muscle pain/cough/sore throat/runny nose. <----- much harder to practice and play a football game in a few days.
End rant.
Both TB12 and Michael Jordan have famously had amazing performances on the field/court while suffering influenza-type symptoms (whether it was actually influenza I have no clue).
If you have a really bad stomach bug, it can be debilitating. You're not absorbing nutrients or liquids, so you're dehydrated, low on electrolytes, and under-nourished. At its peak you can't do anything...I doubt the players are out there shitting themselves and vomiting on the practice field, while light-headed and weak. And it can certainly affect a person for more than 24 hours.
I've had 1-day bouts of the flu, and 8 days of bed rest flu. And I've had brief stomach bugs and 'can't get out of bed for two days' stomach bugs and 'functional but had bouts of vomitting for a week' stomach bugs.
Anyway, all of this is to say: 'harder to practice and play in a few days' is pretty subjective.