The project manager should initiate the change request to modify the scope and adjust the timelines. This is because the project manager is responsible for managing the project scope and ensuring that the project delivers the agreed outcomes within the approved budget and schedule. When the project manager encounters scope changes and a delay in the delivery of critical tasks, they should follow the change management process as defined in the project management plan. This process involves initiating a change request that describes the proposed changes, their impact, and their justification. The change request should then be submitted to the appropriate authority, such as the change control board or the steering committee, for review and approval.
The project manager should also update the project documents, such as the scope baseline, the schedule baseline, and the cost baseline, to reflect the approved changes. The project manager should communicate the changes and their implications to the relevant stakeholders, such as the project team, the PMO, the sponsor, and the customer. References: PMBOK Guide, 6th edition, section 4.6, page 115, PMP Exam Prep, 10th edition, page 180