The club's doctors visited Messi at his home in the morning, where they gave the Argentina ace medication and advised him to rest.
Barca announced on their website that the record-breaking forward had "gone down with a fever and is feeling generally unwell".
Barca have a day off on Thursday but will be hoping Messi can take part in Friday's final training session before their re-match with Real Madrid on Saturday.
Midfielder Xavi, who like Messi started Tuesday night's Copa del Rey semi-final defeat to Real, was also absent from squad training on Wednesday as he received physiotherapy.
The light of the evenstar fades...the time of the elves has ended..- allstarbramble