In this proceeding, we present a proof of principle study for estimating the number of muons of inclined air showers proportional to their energy using hybrid radio and particle detection. We use the radiation energy of an air shower to estimate its electromagnetic energy and measure the muon number independently with the water-Cherenkov detector array (WCD) of the Pierre Auger Observatory. We select 32 high-quality events in almost six years of data with electromagnetic energies above 4 EeV to ensure full efficiency for the WCD reconstruction. The muon content in data is found to be compatible with the one for an iron primary as predicted by current-generation hadronic interaction models. This can be interpreted as a deficit of muons in simulations as a lighter mass composition is expected from Xmax measurements. Such a muon deficit was already observed in previous analyses of the Auger collaboration and is now confirmed for the first time with radio data. Currently, this analysis is limited by low statistics due to the small area of AERA of 17 km² and the high energy threshold. We will outline the advantages of using radio detection instead of the Auger Fluorescence Detector in future analyses allowing for high-statistic measurements of the muon content as a function of energy.