Marc Wilmots refuses to rule out Vincent Kompany
Captain Kompany (abdominal, groin) and Vermaelen (hamstring) are rated doubtful for the last-16 clash with Jurgen Klinsmann's side in Salvador on Tuesday.
Kompany missed Belgium's Group H finale against South Korea after playing a full part in their first two matches in Brazil, while Vermaelen has managed just 31 minutes of the campaign so far after being substituted in the first half against Russia.
Wilmots is no closer to deciding whether the Premier League pair will be able to return against the USA.
"It is impossible to say now whether Vermaelen and Kompany get fit for the United States," the coach said.
"I am pleased that we now have more preparation; we will have to wait and we need to look at their availability day by day. Kompany is struggling with an overload on the abdominal and groin.
"Kompany and Vermaelen continue their individual programmes.
"They have a plan, and Thomas has already trained. So everything is going according to plan."