The playmaker, 29, suffered a hamstring problem on Saturday while playing for Queens Park Rangers during their 1-0 win over Derby County in the Championship playoff final.
Kranjcar has now been told that he will be sidelined for at least a month before he is ready to return to action.
"Niko Kranjcar has ruptured his hamstring," Croatian team doctor Boris Nemec told Jutarnji list. "The rupture is some inches wide and will need between four and six weeks to heal."
Kranjcar has scored 16 goals in 81 games for Croatia since he made his debut in 2004.