The Shakhtar Donetsk midfielder was hurt under a challenge from Casemiro as Brazil training at Tottenham's facilities on Thursday and he sat out the rest of the session with his ankle strapped up.
Fred didn't train with the rest of the squad on Friday and the Brazilian federation offered no details on the extent of his injury, which certainly rules him out of their next friendly against Austria on Sunday.
Reports in Brazil now claim there is a "climate of concern" over the 25-year-old, who agreed a £52million move to Old Trafford earlier this week.
On Thursday, Brazil's chief medical officer, Rodrigo Lasmar, was very cautious when talking about Fred.
"It is early to make any diagnosis before the first 24 hours," he told a news conference.
Lasmar also said that it would only be possible to assess the full picture on Friday, after a scan, but it is also possible that Brazil could take Fred with them to Vienna this weekend and scan the injury there.