Injuries and a lack of pace have meant Torro has struggled in the Bundesliga, making just 15 appearances.
And he is close to an Osasuna return with a €2.5m transfer fee under discussion.
Eintracht want to include a sell-on clause in the deal for Torro, a former Spain youth international.
Torro made 39 appearances for Osasuna, who he joined after five years in the Madrid youth system.
Osasuna have now confirmed the return of Lucas Torro from Eintracht Frankfurt on a four-year deal.
The transfer fee is €2m with an €11m release clause written into his contract.
— C. A. OSASUNA (@CAOsasuna) August 4, 2020