DENVER – When your third line features two players, Wayne Simmonds and James van Riemsdyk, capable of reaching the 30-goal mark, you can understand why the Flyers offense is expected to carry them this season.

The Flyers should score a ton of goals, but there are questions about the defense. Big questions. That means the development of second-year players Robert Hagg and Travis Sanheim will be a major key – and could be the difference between a good season or a memorable one for a team chasing its first Stanley Cup since 1975.

As a rookie, Hagg, a physical defender, was mostly productive until slumping late in the season and being benched by coach Dave Hakstol. The puck-moving Sanheim was ineffective early in the season, but after being sent down to the AHL's Phantoms for some seasoning, he returned to the Flyers and showed marked improvement.

Both players were outstanding Thursday in a 5-2 opening-night win in Las Vegas. Hagg had the first two-point game (goal, assist) of his career and added six hits. Sanheim set up Oskar Lindblom's momentum-changing goal to tie the score at 1-1 and, like Hagg, played solidly on defense. Both players were plus-2.

"It's their second year, and in your second year, you're definitely more comfortable coming into camp," said Ivan Provorov, who is in his third season and is the anchor of the Flyers defense. "You know the system, and just being comfortable helps you take a huge step forward."

Hagg, 23, agreed.

"I'm a little bit less nervous this year," he said. "I know what to expect from myself now and what my teammates expect from me, too, so that should help."

For Hagg and Sanheim, last season was "part of their growth," said Hakstol, whose deep farm system has two other young defensemen, Phil Myers and Samuel Morin (out until February with a knee injury), close to being in the NHL.  "Each of them had some peaks and valleys through the year, and that's to be expected from a young defenseman.

"The experience for them was good, and I really liked the way both of those guys mentally worked through things. They showed toughness, they showed some grit – not just physically, but mentally. That's going to pay off for those guys this year."

Added Hakstol: "There's still challenges ahead for them. It's a tough league for young defensemen, but those guys are going to be in key roles for us and playing in key situations. But I expect to see them continue to grow and embrace those roles for us."

Sanheim and Hagg will be paired with veterans this season. In the opener, the 6-foot-2, 204-pound Hagg was with Andrew MacDonald, while the 6-3, 181-pound Sanheim was with Radko Gudas.

"I think we just need to find our confidence, our consistent game," Gudas, who struggled last season and didn't play Saturday in Denver, said of being together with Sanheim. "I think we can be a pretty useful pair, more defensively. And he's able to create some offense and help the forwards, too."

Sanheim and Gudas were defensive partners for a while last season.

"We're familiar with each other," Sanheim said. "It's nice to have his physical presence and he's so good defensively. He definitely helps me out."

After returning from the Phantoms and fine-tuning his game, Sanheim said, "I just wanted to come up here and play similarly to the way I was playing in the American League. Just playing my game, have the puck on my stick and make plays."

Sanheim said he is "more confident that I can play in this league and do well in this league. I'm just looking forward to building on that and trying to get better each day."

Provorov, 21,  and Shayne Gostisbehere, 25, are young veterans who form the top pairing and will eat up big minutes and be matched against opponents' best lines, when possible.

"But if you want to be a successful team, the second and third pairs are important, too," Gudas said. "They have to be able to create some offense and shut down teams, and when you're on the road, you don't get the second change, so we might see" some high-scoring lines.

All of the defensemen have formed a bond, Hagg said.

"We've having a lot of fun outside the rink, and I think that shows on the ice, too," Hagg said. "Most of us are in the same age group and we help each other out. Like if someone is having a tough night, someone will push him in the right way and you don't take it bad."