User blog comment:B214/Light Novel Volume 16 spoilers/@comment-75.93.25.66-20140105180833/@comment-16281343-20140115092240

If its whether he purposely didn't use it or he can't, I'd go with the former, imo.

Reviving Regulus with 7 is more practical than reviving him with a mutation piece.

Regulus (7 Pawn) + 1 mutation piece > Regulus (1 mutation piece) + creatures worth 7 Pawns.