Barlow did not recover from minor surgery on his jaw after a head knock during last Sunday's win over North Melbourne, while Mundy still hasn't overcome the calf problem that saw him withdrawn from the Kangaroos clash.
"I think it becomes incrementally harder but we've shown great capacity to lift up and everyone accept challenges given to them," Lyon said at Perth Airport.
Fremantle erred on the side of caution with Mundy's calf.
"Calves can be really short term or if you muck around with them they're really quite dangerous," Lyon said. "Rather than push him through and take a risk, we as a collective are getting him up for next week.
Lyon believes that his side is still able to cover the loss of the star duo.
"We know we've got a lot more through the midfield now with (Matt) de Boer, (Nick) Suban, (Paul) Duffield and (Garrick) Ibbotson," Lyon said.
"We're a lot more flexible team and at a pinch (Hayden) Ballantyne and (Michael) Walters,"
Fremantle has also included 19-year-old midfielder Tom Sheridan in the travelling squad.
Sheridan has played just one match for Fremantle since his selection at No.16 in the 2011 draft, against Melbourne in round 23 last season, and has averaged 22 disposals in 11 WAFL matches this season.
"We think it's a wonderful challenge for him," Lyon said.
"He's a part of our youth and gets to experience what greatness looks like.
"(James) Kelly, (Joel) Selwood, (Harry) Taylor, Taylor Hunt and (Corey) Enright and Joel Corey, they're great players so he gets to experience that and we expect him to compete."
Fremantle has won just once in 12 attempts at Simonds Stadium, and has not played there since 2009, but Lyon was not fearful of the task.
"I think it's just a challenge going down there. We haven't beaten them there for a while," Lyon said.
"Under lights is something different. But we like to be an anywhere, anytime team and we know past results count for nothing."
Lyon did not glean much from Geelong's last quarter collapse against the Brisbane Lions last Sunday.
"What I learnt is it's a really dangerous competition. If you lapse, you get hurt," Lyon said.
"I'm more focused on the 52 points up.
"I just think they got it 95 per cent right and they just, for whatever reason, lapsed.
"They're a physical team, they hunt opposition players and they protect their own. So we welcome it, and it's going to be difficult."
