McT, for his warts, is at least a middle 6 winger on the NHL today, and will be paid as one in a year or two time.
Nemec is a worse skater than McTavish, is equally or more lost in his own zone than McTavish is, and a team who drafted him 2nd overall was so unimpressed with his trajectory that they played Peaches and Kovalcevic over him when the could, and he never tried a PP slot away from Dougie or Hughes except during Dougie's administrative sitting to try and force a trade with him.
Can Nemecs career still be salvaged? Maybe, but not if he wants more than 3-4M AAV as a bridge to figure out if there is, in fact, anything there, and not at the cost of Mctavish.