National team doctor Rodrigo Lasmar said Fred suffered "ankle trauma" after collapsing under a sliding tackle from teammate Casemiro, reports Xinhua news agency.
"Tomorrow we will see what happens and decide if we need to carry out any exams but at the moment it's too early to say anything," Lasmar told reporters on Thursday.
Brazil are preparing for football's showpiece tournament in Russia with a training camp at Tottenham Hotspur's new sports complex in Enfield, north London.
The incident came just two days after Manchester United announced they had reached a deal to sign Fred from Ukraine's Shakhtar Donestsk.
The English club reportedly paid 55 million euros to secure the 25-year-old on a five-year deal.
Brazil begin their World Cup campaign against Switzerland in Rostov on June 17 and will also meet Costa Rica and Serbia in Group E.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
