Yeah, its a bit of a concern.
I've not seen a ton of Dallas lately, but when I've seen them in the POs Zucc has also been more average. When he went there he was more or less literary playing the best hockey of his career, and he carried that right into his first game when he was awesome. But looking at the time he was out with his broken arm, i.e. definitely in the later parts of the period a player usually is out with that injury, its obvious that it was a serious break. He isn't back to full speed yet, and that peak form is just not something that you can assume a player can get back to after a somewhat long injury.
Its a steep price to give up for them like you point out.
OTOH, I wouldn't at all rule out that Zucc was pretty reasonable in their expectations, and he does fit in really well with the vets. Lets hope that he has a great series against STL!