The constraints that physical rehabilitation exergames (PREGs) may impose on Player Experience (PX) evaluation should be identified from a physiotherapists' perspective. In this paper, we present the results of a qualitative study to identify the characteristics and constraints that are relevant to evaluate PX in PREGs. The study included semi-structured interviews conducted during two sessions with three physiotherapists from a local hospital. The collected data was analysed using the thematic content analysis method. The findings indicate that the PX evaluation constraints are related to (a) the rehabilitation context in which PREGs are employed; (b) the pursued rehabilitation goal (i.e., the capability of a PREG to assist the achievement of a rehabilitation goal); and (c) the characteristics of target patients, which may affect their experience and willingness to play. The findings of the study contribute to a comprehensive understanding of PX in PREGs. We concluded that the three groups of constraints may impact the three constructs of PX; i.e., context (rehabilitation context), player (patient) and game system (PREG system). Confirming the need to propose or extend PX models of entertaining games for the case of PREGs.