He went from a guy who gave you 10 rebounds per 36 to a guy who gives you less than 8 rebounds per 36. His defensive and total rebounding percentages are down the gutter. It's not a coincidence this happened after two major injuries, and it's not a coincidence that his minutes are down to 30-33 per game.
He's actually averaging 15 and 7 this season, not 17 and 8, and this is not great production based on what he used to be capable of. Sure, he's still a nice player, but he's damaged goods and will be on the wrong side of 30 for his next contract (should you choose to keep him).
So yeah, half a seasons guaranteed of Al Horford is not worth more than a low first-rounder to me, and it shouldn't be to you or to Danny Ainge, either.