Mikel joins up with Nigeria
The midfielder has been out of action since a minor operation last month and missed Chelsea's run-in to the English Premier League title as well as the London club's victory in this month's FA Cup final.
"He has completed his rehabilitation at Chelsea," a Nigerian Football Federation spokesman said.
Nigeria are training near Maldon in the English county of Essex and play a friendly against Colombia at Luton's Knilworth Road on Sunday.
Nigeria's opening World Cup Group B match is against Argentina in Johannesburg on June 12.