Mechanical degradation by shear is one of the mechanisms that limits the life-time of lubricating grease in a rolling bearing. It results in a change of the micro-structure of the thickener-oil system, leading to a change in bleed and consistency. The resistance to degradation can be quantified using a grease worker. In this paper a novel approach is described, where the grease worker is used to measure the rheological properties of a grease in-situ, while aging the grease. Mechanical energy and pressure losses are calculated using the measured force that is required to push the plate with holes through the grease in the cylinder of the grease worker. A pipe flow model in combination with a power law rheology model are used to analytically determine the power-law fluid consistency index and exponent while aging the grease. The model validity is verified using CFD simulations and measurements using a rheometer with plate-plate geometry.