Honestly though, when did he work in that capacity? Certainly not his first year when he got bumped up because of Duchene's struggles/move to MacKinnon's wing and he had that 50+ point year. Not the first year with Bednar when he damn near got booted off the team entirely and played the worst hockey of his career in a 4th line role (and I still think that "descent" began when Bednar bumped him down the lineup and he kinda/sorta quit before the team as a whole did). And it wasn't last year either, because the Nieto-Soda-Comeau trio was the team's de facto 2nd line following the Duchene trade.
Everyone can feel free to disagree with me, but I still think Big Carl's defensive acumen is nowhere near as good as his reputation suggests. He's okay defensively, not great. His value comes in a secondary scoring role. He's got a good shot and can create chances down low/in front of the net. And that's why I don't think he ever has, and never will, work as a 3C/4C. That's why I'm hoping they deal him before his deal expires. I've always considered him an interim guy.