I'm not a doctor so I have no idea what the deal is, but it certainly seems like this injury is taking forever to heal. He did not look comfortable AT ALL when he tried playing early this year.
But I'm not sure it matters in regard to the Drummond talk. Unless you're going to be a franchise that pays no mind to the luxury tax, you can only allocate so many dollars to payroll, and it's always good to have a pipeline of good and cheaper replacement talent.