Merrett injured his ankle in the first quarter of last Saturday night's match against Adelaide at the Gabba.
Fellow defender Joel Patfull has undergone surgery on his broken jaw and is set to miss 4-5 weeks.
Meanwhile, Lions midfielder Troy Selwood has also been ruled out of this Sunday afternoon's clash against St Kilda with concussion after not satisfying the Club doctors' medical test.
Please click here to view the Club's full injury list.