According to a tweet from the Nigeria Football Federation (NFF), the lanky forward has been drafted into Gernot Rohr’s squad for the games taking place in November.
Simon, 25, suffered a groin injury in Saturday’s 3-0 defeat to Paris Saint-Germain in a French top-flight clash – and was subsequently replaced by Kader Bamba after 25 minutes.
Although Nantes are yet to announce the number of weeks he would spend on the sidelines, a reliable source very close to the player said the ex-Levante man would be unavailable for the showdown against John Keister’s men.
“Moses [Simon] was unable to go for a test today [on Sunday] to know the severity of his groin injury, but I can confirm to you that he won’t be available for Nigeria’s games against Sierra Leone,” the source who pleaded anonymity told Goal.
“He will commence treatment as soon as possible and he will return stronger and better in the coming weeks.”
Onuachu scored his ninth goal of the season in his tenth appearance for Genk as they silenced KAS Eupen 4-0 in Friday's First Division A clash.
He was part of the Super Eagles team that faced Algeria and Tunisia in October's international friendlies staged in Austria, albeit, he was unable to find the net.
The three-time African kings take on the Leone Stars at the Samuel Ogbemudia Stadium, Benin City on November 13 before heading to Freetown for the return leg four days later.
With six points from two games, Nigeria lead Group L with six points and victory in both games will assure the three-time Africa champions a place at the 2022 Afcon in Cameroon.
Defenders: Kenneth Omeruo (CD Leganes, Spain); Leon Balogun (Glasgow Rangers, Scotland); Chidozie Awaziem (FC Boavista, Portugal); William Ekong (Watford FC, England); Olaoluwa Aina (Fulham FC, England); Jamilu Collins (SC Padeborn 07, Germany); Oluwasemilogo Ajayi (West Bromwich Albion, England); Zaidu Sanusi (FC Porto, Portugal); Kevin Akpoguma (TSG 1899 Hoffenheim, Germany)
Forwards: Ahmed Musa (Unattached); Alex Iwobi (Everton FC, England); Emmanuel Dennis Bonaventure (Club Brugge, Belgium); Paul Onuachu (Genk, Belgium); Samuel Chukwueze (Villarreal FC, Spain); Victor Osimhen (Napoli FC, Italy); Kelechi Iheanacho (Leicester City, England); Chidera Ejuke (CSKA Moscow, Russia)