The Tottenham star, 23, has had a stomach bug in the past few days and also took a knock on his ankle in the 2-1 comeback win over Scotland.
Bale played the first half on Friday and faces a battle to be ready to face Croatia on Tuesday.
Boss Chris Coleman said: “Wrap him in cotton wool is all we can do.
“If he’s not ready we can’t risk him. We have to be fair to Tottenham.
“I’m not sitting on the fence. The next 24 hours are crucial.”