Ultimately, when GMs sign previously injured players, they know they are taking a risk and rely on what the doctors say. They weigh the chance or further injury, especially as related to the previous one vs what the player might produce when healthy or mostly healthy.
In Sale's case, I think DD did the right thing. He paid Sale 4 years of pay on a 5 year contract, thinking he'd likely miss a year. Had Sale only missed one year and pitched well the other 4, we'd all be happy. 4 years of vintage Sale is worth $145M, easily, in today's market.
Any player can get hurt at any time, but the ones who re-injure past and known injuries seem to stick out as dumb signings. Nobody calls the 2022 Verlander signing a mistake, despite his injury history. Some called the Nate 4 year deal a mistake, and maybe it was. He did miss some time, but probably earned that $68M over 4 years.
GM are always the fall guys, It's the nature of the business. They are expected to be fortune tellers and be able to predict decline, inclines, injuries and career years. It's totally understandable why they get the blame. They usually make the final decision, unless a ton of money is involved.
One can make a strong case against some of DD's choices- like the Sale extension and not locking up our younger players earlier, but I think he did a great job signing players, trading some over-rated prospects for useful pieces and to me, extending one of the greatest SP'ers in the previous decade to a $145M/5 year deal, despite the injury risk, was totally understandable. Of course, in hindsight we can say, why didn't he just wait a year, but had Sale won the Cy Young, he's have cost $250M/6, and we'd b e bitching about why we let Sale go to free agency. It's a now-in job these GMs have, and when they win it all, they usually don't get too much credit or praise.