Digital Behavior Change Interventions (DBCIs) can support the development of new health behaviors. Evaluating their effectiveness is crucial for improving them and understanding the factors that contribute to their success. Building on the CAncer PAtients Better Life Experience (CAPABLE) project, this study proposes both a conceptual framework mapping the relationships between patient motivation, engagement, and outcomes, and a set of testable hypotheses to evaluate these relationships. The framework and hypotheses are specifically designed for small-scale studies, which we demonstrate through an implementation aimed at supporting cancer patients in maintaining their physical functioning during treatment. We find that acting on patients’ initial motivation through clinician prescriptions increases sustained engagement with DBCIs, and that engaging with a DBCI once a week is sufficient to maintain well-being.