Mediocrity cannot be remedied, unless you are a big company where you have dedicated departments to cater for these sort of people ( and by the way, they aim to accomplish the same outcome, just a little bit slower and more paperwork).
All I am saying is, an employee has no business performing a job he/she cannot do unless he is a trainee with a good attitude.
I leave this comment section with one last nugget of wisdom :
There are more skilled and deserving individuals looking for jobs that they can't get because a bad manager is holding on to a mediocre employee.
If an employee veers off course and you wait until you're ready to terminate them before you say anything, you're a bad manager.
And it's not always them, it's sometimes you. Have you never thrived under one manager, been reshuffled and then suffered under another?