According to PMBOK guidelines, when a stakeholder raises a concern about a deliverable, the project manager should first review the project requirements documentation with the team to ensure that the deliverable meets the agreed-upon project requirements. This is crucial because it helps to identify if the concern is valid and if the project is aligned with the stakeholder's expectations.
References: The answer is based on general best practices as outlined in the PMBOK guide, which emphasizes the importance of aligning deliverables with project requirements and stakeholder expectations.