An intern replaced a critical resource mid-sprint and was mentored by a senior engineer. Two months later the project hit critical status and the release milestone slipped by a month. What should the project manager have done to avoid this?

The project manager should have requested an experienced resource as the replacement. When a critical resource leaves, replacing them with an intern, even with mentoring, introduces significant risk due to the intern's lack of experience and domain knowledge. An experienced replacement would have minimized the learning curve and maintained productivity, preventing the project from hitting critical status and slipping the release milestone. Negotiating additional time might be a reactive measure but doesn't address the root cause of the resource gap. Allocating more time for mentoring is good practice but doesn't compensate for the initial lack of experience in a critical role. Convincing the critical resource to stay is often not feasible or within the project manager's control once they've decided to leave.