I think you have to be careful here. Yes for the first 15-20 seconds of a shift, zone starts matter. But last year Carlo had over 1000 shifts starts on the fly and only a couple hundred in the D zone. It matters, but it’s also easy to overstate since so little of the game is played like that.
I would throw out there that Carlo doesn’t drive possession positively at all and the Corsi difference between the rest of his career and the last season + 10 games can be totally explained by Bergeron going away. Over Bergeron’s last 3 years, Carlo had a Corsi over 60% when Bergeron was out there and under 50 without Bergeron out there. And now in a Bergeron free world…here we are.
It’s also not really his job to drive Corsi or possession in general, but to be a steady Eddie.
Team has too many of those.
But the team is also painfully shallow on RD.
The post I was responding to referenced corsi, so I was attempting to give some context around that. But yes like everything in hockey no stat in itself gives you the full picture. But the basic point made was that the way a player is used, including their zone starts and time, will impact their numbers and performance in other areas. As Carlo has been used more and more in a largely shutdown role by Monty, this is reflected in the stats. His on ice corsi percentage goes down, whereas his xGA has gone up (which BTW is also true of the Bruins as a whole under Monty, even when Bergy was still playing). This, to some extent at least, is only to be expected.
That's not entirely to excuse negative trends, it's just to provide context. Other factors are of course also relevant, one of which you referenced. If Carlo's basic job is to help keep the puck out of the Bruins' net, then he has regressed in that regard over the last year+, especially when considering xGA, but so has the rest of the team. Essentially he's contributing to that regression, with caveats, but no more so than the average.