In the real life English top division, there are 1,400+ injuries per season.
In Football Manager 2018, the English top division sees an average of 1,120 injuries per season. That puts FM18 at 80% of reality. 10%-15% of these are considered ‘Major’ (28+ days). The training/match split is roughly 60/40. This is adjusted based on the league in which you choose to manage.