So, what am I supposed to think? Either you don't know who Jake McCabe is, or you don't know who Rick Nash is. It's hard to tell, but since this is wildly out of balance for the Sabres, I will assume the former. Jake McCabe is, right now, arguably the Sabres 2nd best defensemen, on an amazing contract for 3 years, and he's young enough (22), that it's reasonable to expect him to get a lot better. So let's just assume that you don't know who he is, and revise your proposal to:
Girgensons + 3rd <=> Nash
Well, now your talking. Nash is still a force on offense, though his defensive zone play is abysmal. He's old enough that this decline in 2 of the past 3 years is concerning. But he is also a star player, which is not nothing. On the other hand, Girgensons is good. He's very good. His down statistical season aside, he was really good in his limited chances playing with top talent. When he played on Eichel's wing, the Sabres scored a ridiculous 3G/60 5v5. A full goal higher then Eichel apart. Not a blip either. They played 260 minutes together, more than any other player ZG played with. He just didn't rack up the points. Rick Nash is a better player, no doubt, but he's on a expensive contract, on the downside of his career, plays LW only, and is only under team control for two years, while ZG is cost controlled, 21 years old, can play wing or center and is a very effective shut down center, and is under team control for at least 4 more years.
Based on that, I reject your revised (by me) trade. I'd rather move forward with ZG than Rick Nash.