An orthopaedic surgical instrument system (10) that includes an orthopaedic surgical instrument adapted to be positioned on a proximal end of a patient's tibia, and a tibial bearing trial assembly (18), and a tibial evaluation component (136), and a tibial base trial component (14) configured to be coupled to the orthopaedic surgical instrument. A tibial evaluation component includes a base plate (120) and a generally Y-shaped posterior buttress (170) extending upwardly from a superior surface of the base plate, such that the posterior buttress of the insert component is configured to be received in the opening of the tibial bearing trial assembly to prevent rotation of the tibial bearing trial component relative to the tibial base trial component.
