I remember where the speculation began about Marchand being upset about Seguin leaving and it wasn't from any sort of reputable source, but yeah, someone says it and people run with it. Typical speculation.
It was alluded to in a thread previous, either a GDT or a PGT, but, much like Bergeron has had moments where he doesn't look good, that line now has a completely different kind of player on it's right wing. Bergeron and Marchand got used to having to shelter Seguin somewhat in defensive situations, where now they've got a guy who can more than handle his own in that respect. Thus comes some adjustment time for both of those guys. It's about changing the mindset about how you've become accustomed to playing. Smart, versatile players are able to adapt to those kind of changes without losing the core of what skills make them good at what they do.
The fact that people speculated and even think that Marchand has the sads because Seguin is gone is lazy thinking. Marchand had to work pretty hard to get to where he is now. He's been awfully clutch in some high-pressure situations in the past and come through in a big way. Top of the line professional athletes in most sports are not going to get hung up on things like this...as a general rule, they're mentally stronger than that because that's how they get to the top. Anyone who thinks otherwise isn't giving Marchand enough credit.
I am glad though that people still feel compelled to question those kind of speculative things. I was worried here for a bit that's not something people were interested in doing.