Well, you are talking about one GM specifically, and one who hasn't had a real season with a full strength roster to work with yet. He's in his second season running the team, and has yet to get a single inning out of Sale.
If JDM bounces back this year, then why not keep him for 2022? We all know they are getting an ace-caliber starter back without spending a penny. Barring extensions, the team might lose ERod, Barnes and Ottavino. all of whom will need to be repalced. Why add DH to that mix as well? Especially if you have to tie up money by sending it with JDM?
If he does not bounce back, no one will want him unless the Sox pretty much pay the bulk of the fare. So why not hold on to him for one last season? At worst, he becomes a placeholder for Triston Casas, allowing the Sox to play those nasty service time games...