The host nation's 2-1 win over Colombia in the quarter-final on Friday was overshadowed by star man Neymar fracturing a vertebra in his back.
He will miss the rest of the tournament, and Willian could now be doubtful for Tuesday's semi-final against Germany in Belo Horizonte.
The Chelsea man has been widely tipped to replace Neymar in Luiz Felipe Scolari's starting line-up.
However, it remains to be seen whether Willian will be able to feature at Estadio Mineirao.
Brazil will also be without suspended captain Thiago Silva when they face Germany, with a place in the final - against Argentina or the Netherlands - at stake.
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