A Physical Rehabilitation Exergame (PREG) is included in physical therapy to assist a therapeutic goal and improve patients' motivation during treatments. Player Experience (PX) aspects, methods and instruments are relevant to assess patients' attitude towards a PREG. Nevertheless, assessing the effectiveness of a PREG to assist a therapeutic goal is relevant since patients' major expectations are to progress and complete treatments as soon as possible. Therefore, the selection of aspects to evaluate PX in PREGs and the instruments and methods to be employed should consider (i) the rehabilitation context, (ii) the characteristics of target patients / players and (iii) the assisted therapeutic goal. In this paper, we present a qualitative study to identify a set of relevant aspects, instruments and methods that allow evaluators to assess PX in PREGs comprehensively. The study was conducted involving experts from the physiotherapy domain. We found that relevant evaluation aspects are related to therapeutic meaningfulness, personalisation, tutorial quality, movement support and therapeutic goal assistance. Moreover, some instruments and methods from the field of physiotherapy may be required to assess the impact of a PREG on a treatment.