Ilicevic has suffered a ruptured calf muscle.
"I am sorry about Ilicevic," coach Slaven Bilic said. "We have lost another important player who is different to most of our team."
Defender Vedran Corluka also has a thigh problem and will go for tests on Friday.
Bilic said: "It should be OK, he just felt something in his muscle. Based on the prognosis it shouldn't be anything serious."
Croatia, who had already lost forward Ivica Olic for the tournament, start their Group B campaign against Ireland in Poznan on Sunday.