Expectations, however, were understandably tempered. McAvoy, while his talents undeniable, had never played in a National Hockey League game and was just a couple of weeks removed from roaming the campus of Boston University.
But after four games, the 19-year-old has done anything but disappoint. McAvoy has been among Boston's best all-around performers in the series, logging an average of 25 minutes, 26 seconds of ice time, second only to Zdeno Chara.
"He's a heck of a talent. He wants the puck. He wants to be out there in big moments. I think the Boston Bruins fans are seeing something right now that they're going to truly appreciate for years," Cassidy said following Boston's Game 4 loss, during which McAvoy had his second assist of the series negated after Noel Acciari's goal was reversed on an offside review.
McAvoy's poise with the puck has been eye-popping, as has his ability to keep up with the pace and strength of players that, in some cases, are over a decade older.
"Even on the [Senators'] goal [in Game 4], he's battling right to the end against a big-bodied guy," said Cassidy. "We can't ask for much more from him. He's come in and moved the puck…even when we got behind there, he's pushing the pace and trying to make things happen, and those are special talents when, in situations like this, they want to be a difference maker.
"They can't teach that. We can teach him some things system-wise that he'll pick up in a hurry. But, the stuff that he has - natural talents and abilities that you're seeing - I think they're getting a little bit better every time we see them."
The 6-foot, 208-pound right shot has also fit in seamlessly on the Bruins' first power-play unit. McAvoy, who played 1:51 of the Bruins' lone power play in Game 4, has slotted into Krug's spot at the point, as the "quarterback" of the man advantage.
In Game 3, McAvoy picked up a helper on David Pastrnak's blast from the circle, when he delivered a slippery feed to the winger for a one-timer.
"Charlie's been great, he's a phenomenal player," said Brad Marchand, who along with Patrice Bergeron, Ryan Spooner, and Pastrnak, makes up the rest of the first unit. "Very smart with the puck, makes a lot of really good plays. He's been good to jump in there, especially losing a guy like Torey - he's not easy to replace. But he's been really good for us."