Lionel Messi did not train with his Barcelona teammates on Wednesday morning after being visited at home by club doctors to treat a fever, the club said.
"The Argentine player is suffering from a fever and general sickness," said a statement on the Barcelona website. "The first-team doctors have visited Messi at his home where they have given him the necessary medication and recommended he rest."
Messi played for the full 90 minutes as Barcelona were unceremoniously dumped out of the Copa del Rey on Tuesday night after a 3-1 defeat at home to Real Madrid.
However, the club are still hopeful that the four-time World Player of the Year could be fit for another clash with Madrid in the league on Saturday.
"This Thursday the team has a day off and it is hoped that the Barca number 10 could still train on Friday afternoon, which will be the last training session before the Clasico in the league at the Bernabeu," the club statement added.
Messi has taken part in every league game this season as Barca have built up a 16-point lead over Madrid in the title race.