In order to position an ankle prosthesis including a tibial implant and a talus implant provided with a talo-calcaneal anchoring keel, the invention proposes a surgical instrumentation assembly including: a tibial phantom (60) of the tibial implant, adapted so as to be attached to the tibia (T) of a patient, and an aiming guide (80) adapted for setting into place an instrumentation element (90) through the talus (A) and the calcaneus (C) of the patient, along an axis (Z-Z) for implanting the talo-calcaneal anchoring keel, the tibial phantom and the aiming guide mechanically cooperating with each other to restrict movement of the tibial phantom and the aiming guide relative to each other along vertical and medio-lateral directions, wherein the aiming guide is configured to guide placement of the instrumentation element.