The Denmark international has netted six goals and provided one assist for struggling Lega across 24 La Liga appearances this campaign.
The Blaugrana will find out if they are allowed by La Liga to sign a contracted player from another Spanish club this month following a long-term injury for Ousmane Dembélé, who will not feature again this campaign.
That followed on from a similar season-ending injury to main striker Luis Suarez, while Abel Ruìz and Carles Pérez were both sold by the Catalan giants last month.
Leganes have already lost star striker Youssef En-Nesyri last month, when Sevilla signed him in a €20m deal.