Assessing work ability is challenging in primary care, and practices vary widely across sickness certification. Such decisions often have major consequences for patients’ trajectories and return-to-work prospects. This study explored physicians’ clinical reasoning when assessing reduced work ability and recommending sick leave, with the aim of clarifying how person-centred decision-making unfolds in everyday practice. Data were obtained from a survey in which 142 physicians assessed six authentic patient cases and described their reasoning for each case. In total, 925 responses were analyzed using qualitative content analysis, yielding four categories. Assessment of the patient’s condition involved balancing medical, functional and social factors in a complex decision process. When objective findings were absent, the patient’s narrative became pivotal, requiring clinical experience and judgement. Sickness certification as a process over time reflected views of sick leave as part of rehabilitation, continuously reappraised in relation to recovery and work ability. Work demands and resources captured the person–work interaction—how demands meet the patient’s resources and limitations—highlighting the workplace as a key arena for adaptation and participation. External resources and support structures emphasised organizational conditions, including access to rehabilitation resources, clear guidelines, and collaboration with employers and other actors. The results align with the principles of person-centred and integrated care by portraying sickness certification as a relational and continuous process in which the patient’s narrative, resources and life situation are central. The assessment is shaped through dialogue between physician and patient, in collaboration with other actors, underscoring the importance of participation, continuity and coordination in rehabilitation. Viewing sickness certification as part of an ongoing care and recovery process is consistent with the pursuit of a more person-centred and sustainable healthcare system.