Barcelona say they are prepared to take their bid to register Turkey midfielder Arda Turan to the Court of Arbitration for Sport (Cas).
The Spanish and European champions are unable to play Turan, 28, under a Fifa ban for breaking transfer rules.
But after an injury to Rafinha, they have asked Fifa and the Spanish Football Federation if they can replace the Brazil midfielder with Arda.
If they are refused permission, Barcelona will appeal to Cas.
They were prevented from registering players for two transfer windows, with the ban officially expiring at the close of the last window in September.
While they would normally have to wait for the new window to open in January to register new players, rules stipulate that in emergency cases such as injury, players can be registered outside the scheduled transfer windows.
The presence of Turan, who signed in July from Atletico Madrid in a deal worth an initial £24m, would be a boost for Barca, who also have Lionel Messi and Andres Iniesta sidelined.