This paper considers the impact of a damaged leading edge (LE) on the stall margin and stall inception mechanisms of a transonic, low-pressure ratio fan. The damage takes the form of a squared-off leading edge over the upper half of the blade. Full-annulus, unsteady CFD simulations are used to explain the stall inception mechanisms for the fan at low- and high-speed operating points. A combination of steady and unsteady simulations shows that the fan is predicted to be sensitive to leading edge damage at low-speed, but insensitive at high-speed. This blind prediction aligns well with rig test data. The difference in response is shown to be caused by the change between subsonic and supersonic flow regimes at the leading edge. Where the inlet relative flow is subsonic, rotating stall is initiated by the growth and propagation of a subsonic leading edge flow separation. This separation is shown to be triggered at higher mass flow rates when the leading edge is damaged, reducing the stable flow range. Where the inlet relative flow is supersonic, the flow undergoes a supersonic expansion around the leading edge, creating a supersonic flow patch terminated by a shock on the suction surface. Rotating stall is triggered by the growth of this separation, which is insensitive to the leading edge shape. This creates a considerable difference in sensitivity to damage at low- and high-speed operating points.