According to coach Bruce Cassidy, McAvoy "hasn't felt well" since taking a hit against Edmonton last week at TD Garden.
"McAvoy said yesterday he wasn't feeling right," said Cassidy. "We contacted our medical staff back home and he's on a plane today to undergo some tests. It's a vague answer because we don't know what's going on. Charlie said he wasn't feeling right, so we want to make sure he gets looked at and we'll go from there."
Cassidy went on to say he did not think the blue liner's ailment was related to the heart condition he was treated for last season.
"I don't believe so," said Cassidy. "I think he got hit against Edmonton - or he did get hit against Edmonton, hasn't felt well. He was trying to play through it…we want to make sure he's doing OK, so that's step one."
Video: Cassidy talks defense shuffle
Miller, meanwhile, blocked a shot with his hand during Thursday's loss to the Oilers and will be evaluated by the medical staff back in Boston to determine the next step.
"I don't know what particular part of his hand," said Cassidy. "So he has to go back and get it further evaluated. We'll probably have a better idea in a few days what the X-ray shows and whether they cast it or if he has to miss time."
With McAvoy and Miller sidelined, Cassidy will turn to rookie Urho Vaakanainen and veteran Steven Kampfer to fill the void.
Vaakanainen, who will make his NHL debut alongside Matt Grzelcyk, played for Providence on Friday night in Bridgeport, Conn., and hopped on a flight from Boston to Vancouver - with a layover in Toronto - early Saturday morning, arriving to Rogers Arena just after pregame skate.
"I assume as a 19-year-old, he'll have lots of adrenaline," said Cassidy. "I don't see that being an issue. Obviously it's more about the time change adjustment, your legs, been on a plane all day.
"But as for the lack of sleep, most guys for their first NHL game - whether they know the day before or not - they probably haven't slept that well the night before."
Kampfer will be suiting up for the Black & Gold for the first time since 2012. The 30-year-old played 48 games for Boston from 2010-12 and went on to stops in Minnesota, Florida, and New York, before being re-acquired as part of the Adam McQuaid trade with the Rangers on the eve of training camp.
"I'm excited," said Kampfer, who will be paired with John Moore. "You don't want to see guys get hurt or whatever, but it's part of the game. Just looking forward to the opportunity and helping the guys play good and hopefully we get a win."
Video: Kampfer ready for first game of season
The 5-foot-11, 198-pound right shot had been a healthy scratch for the Bruins' first seven games, a role he was prepared for heading into the season.
"I thought I had a pretty good preseason," said Kampfer, who will also man the point on Boston's second power-play unit. "But I kind of knew the situation going in and Bruce and [Don Sweeney] and the coaches kind of talked about what the role was gonna be like going into the season.
"I kind of knew that I was gonna have to wait for something like this to happen. But it's just [about] staying ready and the coaching staff has done a great job. Just looking forward to tonight."
Cassidy said he is prepared to juggle his defense pairs if need be, particularly outside of the top duo of Zdeno Chara and Brandon Carlo, which has been a strong shutdown pairing in the past.
"Zee and Carlo have played together, so we're comfortable there," said Cassidy. "The other four, it might be a trial and error thing…[Vaakanainen, a left shot] has played the right for us in camp a little bit, so has Moore. We may juggle those as we go."
Regardless of how things line up, the Bruins are prepared to deal with their first real adversity of the young season by adopting a "next man up mentality," as they did so many times a year ago.
"It's always about the next man up," said Patrice Bergeron. "There's opportunity for them to step up and bring something to the table obviously. That's how we operate. It's about everyone chipping in and trying to contribute."
Video: McAvoy and Miller are out, Kampfer and Vaakanainen in
Another Familiar Foe
For the second time on this trip, the Bruins will be facing a former teammate for the first time. After taking on Austin Czarnik in Calgary, Boston will be up against Tim Schaller, who signed a two-year deal with the Canucks in July.
"It won't be strange. It will be fun," said Schaller, who had a two-assist game against Pittsburgh on Tuesday for his first points of the season. "A bunch of good guys over there. Looking to battle and compete against them and definitely beat them."
Schaller, who played in all 82 regular-season games last season, helped form one of the best fourth lines in the league, setting careers highs in goals (12), assists (10), and points (22) while playing mostly with Noel Acciari and Sean Kuraly.
"Very good. Filled a roll for us," said Cassidy. "He played with Kuraly and Acciari a lot, did a real good job, O-zone, puck possession. Played against top lines most nights, lot of D-zone starts, very good on the kill. Good team guy, stood up for this teammates….did a real nice job for us.
"Well liked in the room, good personality. I said before, we'll miss him, but good for him. I wish him luck here, not necessarily tonight - but he's a good person and a good player."
Vancouver is 4-3-0 this season and coming off a 4-1 loss to Winnipeg on Thursday night. The Canucks will be without rookie sensation Elias Pettersson, who played two games before being injured against Florida last week.
Alex Edler, Sven Baertschi, and Bo Horvat are leading the Canucks in scoring with five points apiece, while second-year forward Brock Boeser is second with four points (two goals, two assists).
Goalie Anders Nilsson is off to a strong start, having posted a 3-1-0 record with a 2.26 goals against average and .925 save percentage.
"They're scrappy, they work hard, they skate," said Cassidy. "The goaltender's been good for them."
Video: Bergeron readies for VAN
Saturday's Projected Lineup
Cassidy confirmed after morning skate that Jaroslav Halak will make his second straight start between the pipes and Anders Bjork will return to the lineup after one game as a healthy scratch.
Brad Marchand - Patrice Bergeron - David Pastrnak
Danton Heinen - David Krejci - Jake DeBrusk
Joakim Nordstrom - David Backes - Anders Bjork
Chris Wagner - Sean Kuraly - Noel Acciari
Zdeno Chara - Brandon Carlo
John Moore - Steven Kampfer
Matt Grzelcyk - Urho Vaakanainen
Scratches: Ryan Donato