Manu was elevated alongside his assistant from the U17 team, Nduka Ugbade, to take charge of the country’s U20 team, the Flying Eagles.
“Manu must not shut the door to other players,” warned NFF director of technical Emmanuel Ikpeme.
“We have not said he should not invite some of his U17 players, but he must give other players a chance to fight for a shirt.
“We have abundant talents in the country and I am sure there are some out there who can make the Flying Eagles team if given the opportunity.”
The Flying Eagles will begin their campaign for the 2015 African Youth Championship to be hosted by Senegal in May.
They are scheduled to face the winners of the clash between Kenya and Tanzania.
Should they advance, they will then face the overall winners of the matches between Swaziland and Lesotho and Angola in August.
Nigeria placed third at the 2013 AYC in Algeria to qualify for the U20 World Cup in Turkey, where they reached the Round of 16.