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."
Defensemen Progressing; Vatrano Banged Up
Cassidy said injured blueliners Brandon Carlo, Krug, and Adam McQuaid are making progress in their recoveries, but did not expect any of them to be ready for Game 5.
"No, I don't think so," Cassidy said during Thursday's media availability at Warrior Ice Arena. "There's guys progressing, certainly. But to say in 24 hours we could have people available, I'd say no."
Forward Frank Vatrano is also dealing with an undisclosed ailment. His status for Game 5 will be revisited on Friday morning.
"He's OK, he'll be off the ice today as far as I know," said Cassidy. "He might test it. So, he'll be a decision tomorrow morning, we'll see how he is and go from there."
Video: Cassidy, players speak ahead of Game 5 in Ottawa
Bergeron A Selke Finalist - Again
On Wednesday, Patrice Bergeron was named one of three finalists for the Frank J. Selke Trophy, as the NHL's top defensive forward, joining Minnesota's Mikko Koivu and Anaheim's Ryan Kesler. Bergeron has won the award three times and has been a finalist for six straight seasons.
"I think you see the obvious, the face-offs, the penalty kill, match-ups. You don't have a true appreciation for how good a stick he has and how his angles and closing are until you see it every day," said Cassidy.
"He just has an uncanny ability to be on the right side of the puck and get his stick in passing lanes, close off breakouts that you're constantly kind of teaching the younger guys how to do it.
"That's the biggest thing, just his hockey IQ when it comes to getting in the passing lanes and killing plays…that's where he's second to none."
A Challenging Situation
The Bruins believed they had found the back of the net in Game 3, when Noel Acciari tipped home a McAvoy point shot midway through the second period. But Ottawa challenged the play and after review, the officials discovered Acciari was offside about 20 seconds before the goal.
Despite the disappointment of having a goal wiped away in a 1-0 loss, the Bruins chose to take a broader view of the decision on Thursday morning.
"What I do like about it is it's black and white, there's no ambiguity," said Cassidy. "Last night, it was offside. You go back however long they went, you can't argue with it. It's not a judgment call that way.
"Goal scoring is difficult in this league and to take goals back on such a small play that sometimes doesn't really have an effect is tough. But that's up for the league to decide."
Boston was on the other side of an offside review in Game 2 when a rush that led to Drew Stafford's tally was deemed inconclusive.
"It's such a thin line," said Brad Marchand. "It works for you at times and it works against you sometimes. Obviously when it works against you, you want to see a change, but when it works for you, then it's a great rule."
Video: BOS Recap: Rask makes 26 saves in Game 4 shutout loss