While it's tempting to look at all levels to get a larger sample size of data, sometimes you have to discount minor league injuries. Quite often, depending on your world, owners may not pay much attention to their minors, may not have enough healthy bodies to fill out their rosters, may not be letting simmy handle active/inactive, etc.
You could have situations where injuries are self-inflicted by minor leaguers being heavily fatigied and continuing to play, which greatly increases the likelihood of their incurring injury. This could skew any correlation that exists between health rating and training budgets by throwing a bunch of bad or misleading data into the mix.