Looks like it's Emily Waldon, a Tigers writer for Baseball America, who broke the news, citing a "source" in a tweet:
I haven't seen any confirmation or official sourcing, either. Chris McCoskey at The Detroit News at least ID'd the source as someone who's not authorized to speak publicly about the decision. That probably means it's AJ Hinch, Scott Harris or Mize.
As a journalist, this is not the kind of thing that is important enough to report with an unnamed source. You can only look stupid if you're wrong, if you're wrong people will criticize you for getting Mize's hopes up, and no one will care if you were first except other beat writers if you're right.
David