While managing a project, what actions should a project manager or team lead do to mitigate the risk of performance degradation in the event a project worker leaves the project prior to his or her scheduled finish? How do project managers reduce the impact of this type of threat?
The first question I would ask is: is this person truly considered "key," does this person really possess such a unique set of skills and abilities that it would be near impossible, or very expensive, to replace? This situation can certainly occur, especially for FOIK projects, but often times I find it is more a false perception of that individual.
Nevertheless, if this is the case, you have a created, or allowed to occur, a single point of failure in your project capabilities. Not only with humans, but in all project systems and processes, a PM needs to identify when a SPOF is growing and mitigate this situation immediately by building in redundancies. For machines, this means you have a replacement machine at the ready, parts inventories, and / or a maintenance schedule. For humans, it means you begin training and preparing someone else to assume the duties of this key person so that, if this person becomes unavailable, you have someone on deck to continue performing.
I also have an aversion to people consistently performing on the outside of the performance curve, i.e., way above average. I know this seems counter-intuitive. But with this type of performance, you are setting yourself up for a SPOF that cannot be easily mitigated, and if truly on the outside of the curve, will not likely be mitigated. Then you have to reset customer / stakeholder expectations and reset performance bars. It's great to take advantage of superior, hyper performance but you need to understand your risks when / if that individual leaves or becomes otherwise unavailable or when that person's performance regresses to the mean.