Jefferson told Fox Sports Ohio after Game 4 on Tuesday night that James got sick from a bug that was going around, which also caused backup Deron Williams to miss a shootaround.
"I know he won't talk about it, so I'll give my big guy a shout," Jefferson said. "Deron Williams missed shootaround this morning because he had like a little bug, really lethargic, had no energy. And I think that's what 'Bron had. And sometimes these little bugs can go around. And [James] was like, dude when Deron didn't show up to shootaround, it kind of started clicking in his head. Because for him it was like, 'I don't know why I was so lethargic, why I had no energy, I had nothing.'
"And so, these little things happen. [There] was no panic. Look, he was lethargic, they hit a bunch of tough shots, if Marcus Smart doesn't go 7-for-10 from 3, then we're not even talking about it."
James scored 11 points and had six turnovers in Game 3, a 111-108 loss to the Boston Celtics. After that game, James said he "didn't have it" but didn't mention being sick.
He was back to form in the Cavs' 112-99 Game 4 victory to take a 3-1 series lead, scoring 34 points with 6 assists and 5 rebounds.