Nice have had their request to complete the signing of Sporting Lisbon's Yannick Djalo rejected by the Court of Arbitration for Sport.
Djalo joined the Ligue 1 club on deadline day of the summer transfer window, but they sent the registration documents to the French League after midnight when the deadline had passed.
Fifa rejected the 4.5million euro deal, which prompted Nice to appeal to the international tribunal, but they have upheld the stance of world football's governing body.
Djalo, 25, has now been told he will not be eligible to play for his new club until after the winter transfer window.
A Nice statement read: "We have always said we would be legalist and respectful towards institutions, so we will comply with this decision."
Nice are currently 16th in Ligue 1 with just seven points from their last nine games.