Leon Balogun failed to train with the rest of the Nigeria squad on Thursday morning, and is now a major doubt for Saturday's World Cup opener against Croatia.
The Brighton and Hove Albion defender was conspicuously missing from the team's last session in Essentuki, and team sources told KweséESPN he had gone for a scan.
The results of the scan were still being awaited as the team left Essentuki for Kaliningrad on Thursday afternoon.
In the event that Balogun is unable to start on Saturday, Kenneth Omeruo will be the most likely candidate to step in as replacement, although coach Gernot Rohr has toyed with both a back three and a back four in pre-tournament friendlies.
Injury denied Balogun a spot in Nigeria's 2014 World Cup squad after the defender broke his foot on his international debut against Mexico in March of that year.
He returned to the Super Eagles fold a year later and has struck up a strong understanding with William Troost-Ekong during Rohr's tenure.
The 29-year-old featured for Hannover 96, Werder Bremen, Fortuna Dusseldorf, Darmstadt 98 and Mainz before joining Brighton on a free transfer following the conclusion of the Bundesliga campaign.
Nigeria play their opening game against Croatia at 21:00 local time on Saturday.
PROJECTED LINEUP
Uzoho, Shehu, Troost-Ekong, Omeruo, Idowu; Ndidi, Mikel, Etebo; Iwobi, Moses, Ighalo