Rielly told his mother, Shirley, who was so excited she went right online to learn more.
"I think the first thing she said was, 'Oh, he's a lefty,' " Rielly said.
If there's a wrinkle to the trade, that's it.
[RELATED: Behind the Numbers: Muzzin's impact with Maple Leafs]
Five of Toronto's top six defensemen are left-handed shots, including Rielly, Muzzin's partner on the top pair for practice at Taylor Sportsplex in suburban Detroit on Thursday. Muzzin will make his Maple Leafs debut against the Detroit Red Wings at Little Caesars Arena on Friday (7:30 p.m. ET; FS-D, TSN4, NHL.TV).
Ideally, you'd have three lefties, three righties, and they'd slot clearly into three pairs. The NHL is so fast that split-seconds make a difference and playing on your backhand can cost you split-seconds. It makes cross-ice passes trickier. It can force you to rim the puck around the boards instead of shoot in the offensive zone.
Rielly is having a breakout season, with 50 points (13 goals, 37 assists) in 49 games, third among NHL defensemen. He has been playing on the left with Ron Hainsey, a lefty. Will moving to his off side disrupt him?
"Great question," coach Mike Babcock said.
Babcock brought up Nicklas Lidstrom, who won the Norris Trophy as the NHL's best defenseman seven times with the Red Wings, the last four while Babcock was the coach in Detroit. Lidstrom was a left-handed shot.
"He said, 'Why wouldn't you put the guy who makes all the plays on his forehand?' " Babcock said.
Babcock never put Lidstrom on his off side.
"It's not perfect," Babcock said. "It's what we've got, and it was what was available, and we're going to make it work."
Of course, this is a nice problem to have.
The Maple Leafs made the trade because Muzzin is what they needed in many ways: a 6-foot-3, 213-pound steady presence with 50 games of experience in the Stanley Cup Playoffs. He won the Cup with the Kings in 2014.
Muzzin, who turns 30 on Feb. 21, can play in all situations and should make the defense better, not only by working with Rielly, who turns 25 on March 9, but by dropping Hainsey to the third pair. Hainsey, who turns 38 on March 24, can log fewer minutes against lesser competition and work with 22-year-old Travis Dermott.
By making this trade now, the Maple Leafs have given themselves 33 games to adjust before the playoffs.
"When you put a bunch of good players together, usually we can pick it up pretty quick, so we'll see how it goes," Muzzin said. "I'm excited, so hopefully everything goes smoothly."
Muzzin has played most of his NHL career on the left, and his partner in Los Angeles was Alec Martinez, a lefty.
"It's comfy for me," Muzzin said.
Babcock also tried Muzzin on the left with Jake Gardiner, a lefty, on the second power-play unit. There is no guarantee Muzzin will be there for the morning skate Friday, let alone the game. At first, Muzzin might take a regular shift and kill penalties but not play on the power play.
"I think the No. 1 thing to do is to get your new people feeling good," Babcock said. "And then if you want to make any changes or do anything different, do it after they're getting feeling good and they're comfortable and they know what's going on."
Rielly, for his part, seems unfazed by playing on the right.
"When me and Ron were together I'd oftentimes end up on that side anyway," Rielly said. "So it's just a matter of being comfortable with your partner, talking lots, making sure we know what the deal is, and over time we'll smooth it out. I'm sure it might take a game or two, but that's expected. …
"It just takes time. We're both pretty excited. I think that helps. I mean, he wants to win. We talked once over the break and we talked a lot today, obviously. He's happy to be playing for a team that's motivated and in a position to win games. I mean, that adds to it as well. It makes it a bit easier, I suppose."
The Maple Leafs have not won a playoff series since 2004 and have the top team in the NHL in their division, the Tampa Bay Lightning, not to mention the Boston Bruins, who defeated them in seven games in the Eastern Conference First Round last season. But they have the fifth-best record in the League (.633 point percentage), and now they have Muzzin.
"I mean, we had confidence in this group going into the break," Rielly said. "I think when you add a player like that it just adds to it even more. I mean, the message that's been sent is pretty clear, and it's an opportunity for this team [to show] that we're up to the challenge."