They didn't extend Sale until Mookie had already turned down their extension offer, they probably had a pretty good idea at that point that there was a strong chance he was gone in 2 years, so that shouldn't have come into the equation when choosing to extend Sale or not. And the deal was a lot smaller than you'd expect a 29 year old superstar ace coming off back to back cy young caliber seasons to get because of the injury potential. If they waited a year and he had another strong year then instead of 5-145 you're looking at a monster deal if you want to retain him, I mean look at what Cole signed for with a much weaker resume than Sale.It was indeed a terrible signing, not because they gave Sale an extension per se, but because they gave him an extension right after a season in which he couldn't really pitch the last two months (used very sparingly) because he was significantly hurt, AND he had another year left on his team-friendly contract (plenty of time to see what he could do coming off an injured season, and then decide), AND they knew they were staring down Mookie's free agency, and would only break the bank for so many bajillion dollar contracts.
That said, of course I hope Sale recovers. Like, by his next start. Or he has the wiles to become an effectively crafty pitcher for the rest of his career. But it was a terrible signing by virtue of its timing as much as anything else.
They took a calculated gamble that they were buying low on an extension, the next 3 years will determine if they were right or not.