The Ajax forward has failed to shake off the ankle knock he suffered in Friday's 0-0 draw in Reykjavik, meaning Eidur Gudjohnsen is likely to start in Zagreb.
The Ajax striker picked up an ankle knock during Friday's 0-0 draw in Reykjavik and, despite intensive treatment, has been forced to concede defeat in his battle to be fit for Tuesday's clash in Zagreb.
"From the beginning I was afraid I would miss out on this match, and it is very disappointing to miss the most important game in the Icelandic football history," Sigthorsson told RUV. "But I will follow it with excitement and am sure that the guys will succeed. And I will be ready for Brazil next summer."
Veteran striker Eidur Gudjohnsen is expected to start in Sigthorsson's absence as Iceland attempts to qualify for a major tournament for the first time in its history, with coach Lars Lagerback confident that the Nordic minnows can pull off a sensational upset.
"I always try to find solutions to problems rather than lament over them," the Swede said. "We have nothing to lose and everything to gain in this game. We know Croatia are a team of exceptional quality and [Mario] Mandzukic is one of the most versatile strikers in the game, but I have plenty of faith in my team's character."