The invention relates to a protective assembly (1) for a limb, in particular the shaft-forming part of a motorcycle boot, which is designed to protect the tibia of the motorcyclist. The inventive assembly comprises: an essentially tubular body (2) comprising a slit, which extends along a longitudinal axis (6) and which is designed to fit around the limb; a rigid tongue (10) which is designed to cover the front part of the body and which extends along both sides of said slit; and a flange (12) which extends transversely to the longitudinal direction between the edges of the slit, said flange being adjustably-connected to the body on either side of the slit so that the distance between the edges can be adjusted. The aforementioned tongue is mounted to the flange such that it can slide freely and the tongue and the body co-operate in order automatically to align the tongue in relation to the slit.