If he did have an MRI, and I agree he should have had one and perhaps did, there is only one reason that it would not have been disclosed to the media. That one reason is that there is some degree of structural damage and they do not want the information made public. Public in the sense that if the Lakers know of some degree of vulnerability, they might further attack and exploit PP's situation beyond the current perception that it was just a mild sprain.
Surely, if he had an MRI and it was clean, there is no reason not to disclose that result.