When an employee has been a traditionally solid performer, but then goes into a tailspin, you need to take action as quickly as possible. Although his decreased performance is your immediate concern, it is not the real problem. Rather, the decline in his performance is a symptom, and you need to try to figure out the cause.
Sit down with this employee and let him know that you are concerned about him and would like to help. You can go over his performance to date, and even compare it to his performance on similar projects in the past. Ask him if there is anything you can do to help, and then just listen.
The slippage in his performance can be due to any number of factors, such as boredom, problems with co-workers, health problems, family problems, or substance abuse. This means that you will need to tailor your response to whatever you learn from him. For example, there may be a need to restructure some of his job, or perhaps the problem is more serious and you will need to consider referring him to human resources, an employee assistance program, or other outside help.
Your employee's behavior can be viewed as a cry for help, and, as an effective manager, you are answering it.
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