There is growing interest in utilizing large language models (LLMs) in the field of mental health, and this goes as far as suggesting automated LLM-based therapists. Evaluating such generative models in therapy sessions is essential, yet remains an ongoing and complex challenge. We suggest a novel approach: an LLMbased digital patient platform which generates digital patients that can engage in a text-based conversation with either automated or human therapists. Moreover, we show that LLMs can be used to rate the quality of such sessions by completing questionnaires originally designed for human patients. We demonstrate that the ratings are both statistically reliable and valid, indicating that they are consistent and capable of distinguishing among three levels of therapist expertise. In the present study, we focus on motivational interviewing, but we suggest that this platform can be adapted to facilitate other types of therapies. We plan to publish the digital patient platform and make it available to the research community, with the hope of contributing to the standardization of evaluating automated therapists.