The powerful attacker has picked up yet another injury, and will be forced to watch Saturday's game from the stands due to the knock
The Iceland international sustained a shoulder problem in training earlier this week, and the Amsterdam side have now revealed that he is not fit enough to make the match squad for Saturday's game.
It is not yet exactly known how long Sigthorsson will be unavailable because of his physical problems, but Ajax are hopeful that the striker will resume regular training action next week.
Sigthorsson scored once in Ajax's 2-2 draw against AZ on matchday one, but was benched for last week's 6-1 victory over NEC for precautionary reasons.
The 22-year-old joined the Amsterdam club from AZ in the summer of 2011, but missed a large part of last season due to an ankle injury.