There is a subtle difference here between what one might call as sametcan put it 'physical' injury and that of 'mental injury'. Both are possible, and sometimes even occur simultaneously.
Injury in this broad sense can be generalised as damage, which affects the functioning and wellbeing of the individual.
So we have to ask in what ways the mental life of an individual can be 'injured'?
Physical injury leading to mental injury:
If we consider that the mind is what the brain does, in a fashion the software, then by damaging the networks of neurons in some way this can produce 'physical' injury whilst also creating the increased likelihood of mental 'injury'. Let us imagine a neurodegenerative process like CJD or Alzheimer's, one can immediately see the dramatic changes in mind/personality and mental life that occur due to neuronal degradation.
Similarly, neuronal destruction or TBI arising from something like pugilistic dementia or sports such as football (both UK and US) one may see the influence of the destruction of neurons via behavioural changes such as impulsivity and aggression. This is mental 'injury' in that it can affect the overall mental health of the individual as relationships may break down along with overall functioning.
This physical set of circumstances may be likened to the hardware of a computer which disturbs the ebb and flow of the software. Although, I'm not describing a dualistic framework of mind here, more something akin to Dan Dennett's theory of mind.
Physical injury leading to mental injury aside, we may also contemplate errors in the program (mind) itself that may lead to 'injury'.
Mental injury in the absence of physical or neurological injury*
This is where I think the word 'injury' is in a sense, a metaphorical even poetic term that can refer to an adverse experience of mental life.
An example might be depression or anxiety which arises in the absence of an external stimulus or persists after the original stimulus has been removed. In a sense rather than 'mental' injury, perhaps a better term is 'psychological injury' which I think captures the injury of the mind easier than does 'mental' injury.
I would argue for this term since psychology according to modern definitions is the 'study of the mind' and mental goes back to the Latin 'mens' or 'mentalis' meaning 'mind'.
I suppose my definition of 'mental' injury or perhaps 'psychological injury' is an acquired psychological or mental state due to physical or psychological causes, which impairs functioning, whether this be personally and/or socially which produces some degree of stress to the individual (or those around the individual)
*One has to be careful here as prolonged mental disorders may change the overall topography of the brain. This can be intuitively seen when one considers how the topography or geography of the brain changes in response to learning. So really what we're discussing is not that a change in brain structure occurs but rather which came first, the physical leading to the change in mental life, or the mental life producing (after some time) a change in overall brain structure.
Oh, it sounds strange. I think injury is something more physical. What do you think?