Leganes general director Martin Ortega has complained that Barcelona's acquisition of Denmark centre-forward Martin Braithwaite is "unfair" and damaging to the club.
Barcelona were granted permission to complete an emergency signing outside of the transfer window by the Royal Spanish Football Federation (RFEF), with Ousmane Dembele set for six months on the sidelines after undergoing surgery on a torn hamstring.
The Blaugrana were only able to target players from Spain or free agents during a 15-day period, with the likes of Inaki Williams, Angel Rodriguez, Roger Marti and Lucas Perez all linked with moves to Camp Nou.
Braithwaite was the man eventually drafted in, as Barca met the €18 million (£15m/$19m) release clause in his Leganes contract on Thursday.
The 28-year-old has signed a deal until 2024 with the Spanish champions, but Ortega has questioned the RFEF's decision to allow such a deal to take place at this stage of the season.
Leganes were powerless to prevent Braithwaite's departure and have no opportunity to sign a replacement, as they continue to fight for their Liga status between now and May.
“We consider that there is a regulation that is unfair, from which Barcelona has benefited. The one who is damaged by this is Leganes,” Ortega told reporters.
Braithwaite could be handed his Barca debut as soon as Saturday, with Eibar set to arrive at Camp Nou.
Leganes, meanwhile, must now make do without a talismanic figure as they prepare to come up against Celta Vigo in a relegation six-pointer on the same day.
Braithwaite has contributed six goals and two assists to Leganes' Liga cause this season, but he was unable to prevent the team from sliding down to 19th in the table.
Without the Danish striker, Javier Aguirre's side could well fall back into the Segunda Division, especially if they fail to pick up a positive result at Estadio Municipal de Balaidos this weekend.
Barcelona will hope that Braithwaite can provide high-quality cover for Dembele and Luis Suarez, who is also unlikely to return to action before the end of the current campaign.
Suarez underwent surgery on a knee ligament injury in January, which had left Quique Setien with only Antoine Griezmann and Lionel Messi to call upon up front.
After Barca's latest outing in La Liga, they will turn their attention to a Champions League round of 16 tie against Napoli at Stadio San Paolo on Tuesday.