West Brom striker Kanu is in danger of missing the start of Nigeria's African Nations' Cup campaign.
Kanu, who had been given special dispensation by Nigeria to delay his arrival until next week, is definitely out of Sunday's Barclays Premiership clash at Wigan.
X-rays have revealed a head injury sustained in Saturday's 1-1 FA Cup third-round draw with Reading at The Hawthorns is in fact a depressed fracture.
Kanu becomes the second Baggies player in just five days to sustain the rare injury as Neil Clement suffered the same problem in the 2-1 home defeat to Aston Villa.
The 29-year-old is still expected to join the rest of the Nigeria squad in Egypt next week, but with the injury taking two to three weeks to heal, he is a doubt for the Super Eagles' opener with Ghana on January 23.
Sympathetic Albion boss Bryan Robson said: 'I feel a little bit sorry for the Nigeria officials because they have been very fair to us.
'It's unusual for an international team to be as flexible as they have been and, unfortunately for them and for us, Kanu has suffered this injury.
'I just want to thank the Nigeria FA and their manager for being so fair to us and allowing him to stay.
'We will always try to be fair with them in the future.'