Brazil superstar Neymar hobbled out of training less than three weeks before the Copa America gets underway.
Neymar – who has been replaced by Dani Alves as captain for the South American showpiece tournament on home soil – clutched at his left knee after attempting a shot.
After limping away from the session, the CBF said Neymar will continue to be observed, with Brazil scheduled to open the Copa America against Bolivia on 14 June.
The Paris Saint-Germain forward spent three months on the sidelines with a foot injury earlier this year.
Neymar – who scored 23 goals in all competitions this season after recovering from the injury – is set to serve a three-match ban for striking an opposition supporter after last month's Coupe de France defeat against Rennes.
Brazil – eight-time Copa America champion which failed to progress beyond the group stage in 2016 – has been drawn alongside Bolivia, Peru and Venezuela in Group A.
Before opening its campaign in Sao Paulo, Tite's Brazil will face Qatar (5 June) and Honduras (9 June) in a pair of international friendlies.