Vermaelen will miss the FIFA World Cup round of 16 clash on Tuesday after Belgium coach Marc Wilmots confirmed the Arsenal defender had not recovered from a hamstring strain sustained against Russia in Group H, while Kompany has yet to train fully with an abdominal injury.
Wilmots admitted Kompany has a 50-50 chance of lining up in Salvador, after the Manchester City defender also missed Belgium's final group match against South Korea on Thursday due to abdominal and groin pain.
"Kompany will train half an hour anyhow and then we will give him the morning off to see if there is a reaction. One chance in two that he makes it," Wilmots said on Monday.
"I know this abdominal injury because I played a year with such a problem."
With Anthony Vanden Borre (broken leg) also unavailable, the doubts about Kompany and Vermaelen's injury mean Wilmots could be missing three defenders against the USA.
Kompany and Vermaelen are also two of Belgium's four most-experienced defenders but Wilmots is hopeful the latter will be fit, if his team advance to the last eight.
"For Thomas the game against the USA is just too early," the coach said.
"When we reach the quarter-finals, the chances are very good that he gets fit by then. Within two to three days, he can resume group training sessions in principle."
Midfielder Steven Defour will also be unavailable through suspension but, in good news for Wilmots, Marouane Fellaini has shaken off a calf injury sustained during training on Monday.
Wilmots added that his starting line-up would not be predictable, after he picked a different XI in all three group games.
"What could we expect in my starting line-up? Surprises. With me there are always surprises," the 45-year-old said.
