The Egyptian Football Association said in a statement on Twitter that coach Javier Aguirre had ordered Salah not to train "so as not to worsen his condition" without giving further details.
Salah, who scored two goals in three games during the group stage, is one of the biggest names in the tournament and an icon in his homeland.
His absence would be a significant blow for the seven times champions, who have been criticised at home for their performances in the group stage despite winning all three games without conceding a goal.
Many Egyptians feel that the current team is a shadow of the side which won three successive tournaments between 2006 and 2010.
(Writing by Brian Homewood; Editing by Hugh Lawson)
