The former Fulham midfielder joined Rayo in June, but it is understood that head coach Paco Jemez was unconvinced by the player's fitness and has therefore allowed the 31-year-old to leave.
"Rayo Vallecano and Derek Boateng have reached an agreement for the termination of the contract between both parties," read a statement from Rayo.
"The player will play for SD Eibar. From the club we wish him well in this new phase of his career."
Eibar also confirmed the deal on their official website.