They also know some employees are not good performers.  There may be a history of poor work attendance, not putting in at work, or the employee may be being performance managed for behavioural issues at the workplace.

When that employee claims that they have been injured in the workplace, and that injury isn’t visible, what should you do?

In short: don't disbelieve them.  

Treat them in a balanced and calm way from the moment of the first report of injury. Treat them as though the injury is real, even when you may suspect otherwise.

But why should you not fight it?

For starters, the injury may indeed be genuine. Injuries don’t just occur to certain personality types. Anyone is susceptible, regardless of any precedents they may have set in their past behaviour. Just because you don’t have visual verification of the injury, it doesn’t mean that the injury doesn't exist or is causing limitations.  

Secondly, if you treat an injured worker disrespectfully or with suspicion, they may build up hostility and resentment towards the workplace, making their eventual return to work slower, if indeed it happens at all.

The person who is not keen to be at work is also more likely to react negatively to your suspicion.  

Reciprocity is a social concept that indicates people are more likely to respond in kind.  In response to positive actions and approaches, they are likely to be cooperative. In response to negative approaches, they are more likely to respond in an uncooperative manner. 

Some people are more likely to 'up the ante'.  If you care for them, they are more likely to do something more intensely positive for you.  And the opposite is also true.  Some people, treated in a negative manner, feel justified in behaving in a more negative manner.  

So the poor performer who has an injury, and then gets treated with suspicion and distrust, is more likely to stay away from the workplace, and blame the workplace.  

If they don’t return to work a legal claim is likely to ensue. If antagonism from the employer towards the employee is established, then the judgement is highly likely to favour the employee. This costs you more.

It’s simple.

Don't start from a position of disbelief, unless you have very strong grounds.  The difficult employee needs more communication and offers of work, and the employer to remain professional and encouraging.