Because that is where rookies excel. It's where Chara excels, too.
"We like the young guys with Zee," Cassidy said after the Bruins' final practice before the Stanley Cup Playoffs. "Zee likes to be the big brother. He relishes that role, I think."
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It's a risk, of course, to expose young defensemen to top defense-pair duty. But that risk is mitigated by a number of factors, by the abilities and smarts of Chara, by the captain's way of communicating and explaining on the ice, by the way a young player can bolster the 40-year-old, especially in getting back on pucks and aiding the transition game.
"I think Zee likes tutoring the young guys." Cassidy said. "He's gotten used to it now and he enjoys it. That's part of his game, that he's a student of the game. … So I think that translates when he deals with young guys."
That much is obvious in the raves he draws, in the ways that Hamilton and Trotman and Carlo and others have spoken about getting a chance to play alongside Chara.
They love it. They know how much it benefits them.
That much was immediately obviously to McAvoy as he skated with Chara in his second official NHL practice, at least after he got over his initial awe.
"It was kind of shocking at first when you're out there moving the puck with a guy like that," said the 19-year-old, the No. 14 pick in the 2016 NHL Draft. "He's just such an amazing player, the leader of this team. He was really good to me today. We talked a lot on the ice, just little lessons here and there. He was telling me to do this, do that, and it goes a long way, it really does."
It's why McAvoy thinks that Chara keeps being paired with teenagers, players less than half his age. It just makes sense, putting them in a position to succeed, even if it would seem to be more prudent to hide them on lesser pairs.
"It's not a surprise why they do that, to put a guy with less experience like myself or, at the beginning of the year, Brandon Carlo, and put him with a guy like that, who just has so much experience, who has such a storied career," McAvoy said. "He knows what he's doing.