Direct device-to-device (D2D) communications between mobile terminals in cellular networks allows operators to offload proximity traffic from the Radio Access Network (RAN) and permits out-of-coverage terminals to maintain peer-to-peer communications. In this study, we consider D2D communications in the context of the Long Term Evolution (LTE) RAN and, in particular, the scenario in which D2D communications share LTE uplink resources. Specifically, we evaluate the performance degradation of the cellular LTE uplink in the presence of interference from in-band (underlay) D2D communications. Through physical layer simulations, we quantify the increase in signal-to-noise ratio (SNR) needed to maintain a certain data rate or coverage criterion as a function of the induced noise rise and under various multipath channel conditions. The results can be used to develop physical layer models for network-layer analyses of D2D and cellular network performance.