Chile coach Jorge Sampaoli is sweating over the fitness of defender Gary Medel for Saturday's World Cup last-16 clash with Brazil in Belo Horizonte.

"If the game were today he would not play. It is a real concern," said Sampaoli of the 26-year-old Cardiff City player who rarely misses a game for his country.

"Because of his character there is always hope as he usually recovers quicker than other players.

"He suffered a muscle strain yesterday and we will see how it evolves."

If Medel does miss out, Sampaoli could be tempted to ditch the three-man defence that he used in the 2-0 win against Spain on June 18 and the defeat by the same scoreline to the Netherlands on Monday, and revert to four at the back.

However, the influential Arturo Vidal will return after sitting out Chile's last group game to rest a troublesome knee and avoid collecting a second yellow card that would have led to a suspension.

"Arturo is ready to play," said Sampaoli. "Towards the end of the season with Juventus, he played in the Europa League when he was only 30 percent fit.

"Today he is still nowhere near 100 percent but we all agree that he has to play."