"I'm now receiving infusions, but it is just not clearing up," said Milutinovic.
If the situation does not improve the veteran coach - who has guided China to their first World Cup finals - could miss the twice-daily training sessions that he conducts.
And his absence at China's first-ever World Cup fixture would be a massive blow to the team.
But Milutinovic has a full squad of players to select from and is determined his side get off the a good start.
"The match against Costa Rica will be crucial because it
sets the tone for the rest of the competition," said Milutinovic.
But the China coach is expecting his opponents to be thoroughly prepared for the match.
Milutinovic has managed four teams at World Cup finals - Mexico, Costa Rica, the USA and Nigeria.
And current Costa Rica coach Alexandre Guimareas played for Milutinovic at the 1990 finals.
"Guima was very important on and off the field in Italy
because of his leadership among the players," said Milutinovic.
"It's very important to have a true professional
like him on any team."
"I respect him a lot, and I know that on match day we
will both play for victory."