The Wigan midfielder has been left out of the squad for Wales' upcoming games against Estonia and Azerbaijan.
Koumas, who is expected to have a hip operation during the summer, produced a poor performance in the World Cup qualifier against Finland in March.
But he was disappointed to be criticised publicly when it was known he was carrying an injury, and left the squad before the match with Germany four days later.
Toshack insists he will consider the 29-year-old in future and has explained his reasons for omitting him on this occasion.
"Jason has not played since the game with Finland, not without his problems," said Toshack.
"I want to have a look at some of the younger midfield players and I have decided to leave him out. Much the same as Carl Fletcher, who has been out of training for a few weeks now.
"The door is not closed for Jason, it is because of his lack of games at club level that he is not selected.
"We had a chat after the Finland game and both decided it would be better for him to get his head down, and start playing regular football at his club and we will take it from there."