Players' union FIFPro has written to FIFA to appeal for Peru captain Paolo Guerrero to be allowed to play at the World Cup.
Flamengo forward Guerrero was given a doping ban from the country's first World Cup for 36 years over cocaine contained in a cup of tea infused with coca leaves.
FIFPro, which has condemned the ban as disproportionate, wrote on Twitter that it hoped for "a breakthrough in the next 24-48 hours."
Earlier this month, the Court of Arbitration for Sport (CAS) upheld an appeal by the World Anti-Doping Agency (WADA) to increase the length of Guerrero's ban.
The 34-year-old was initially banned for 12 months after testing positive for benzoylecgonine, a metabolite of cocaine, following Peru's World Cup qualifier against Argentina in October.
FIFA reduced the punishment to six months, which would have enabled him to play in Russia, and he was named in Peru's provisional squad.
But CAS increased the suspension on the grounds that Guerrero bore "some fault or negligence" for the positive test and must face a ban of between one and two years under FIFA's anti-doping rules.
The court said its panel accepted that Guerrero did not intend to drink contaminated tea, and that the drink was not performance-enhancing.
