Hayden has been ruled out for up to half of the 2011 season after having surgery on his fractured left foot last November, and Mzungu has been impressive when filling the veteran's rebounding role in match simulations.
The 24-year-old, who was pre-listed by Gold Coast and then sent to Fremantle during trade week last year, has been a standout runner this pre-season and Duffield said he was suited to the half-back role.
"I think they've been playing Tendai Mzungu back a little bit, and he's obviously a bit more mature and [has] played some really good senior WAFL football," Duffield said from Fremantle Oval on Monday.
"He's fitted into that role really well and obviously has been doing really well in our running.
"We've got a lot of good players that could come in and fill the role and hopefully play somewhere near Roger's ability - we know how important he is for us.
"We've certainly got some younger guys too that have stepped into the role and played pretty well."
Mzungu, who polled 20 Sandover Medal votes last year and won WAFL club Perth's best-and-fairest award, has played as a half-back in the WAFL, also pushing into the midfield.
The athletic utility averaged 28 disposals a game for Perth in 2010 and made an immediate impact at Fremantle, finishing second in the club's opening 3km time trial.
As well as Hayden, Fremantle will this year be looking to replace impressive young midfielder Anthony Morabito, who will miss the entire season after undergoing a knee reconstruction.
Duffield said Morabito's potential for improvement was "scary", but Fremantle had plenty of youngsters that would push to fill his shoes when the club launches its NAB Cup campaign at Patersons Stadium on Sunday.
Duffield himself shapes as midfield option for coach Mark Harvey after successfully pushing up the ground for periods last season.
The 26-year-old said he had slimmed down over the summer and would again offer Fremantle an option in defence and the midfield.
"I'm just a little bit lighter this year - that's probably in anticipation of the interchange rule change and just making sure I'm nice and fit and ready to go," he said.
"I guess the understanding is we're going to be down that one rotation and it might pay to have a bit more running in you versus a bit more size as the games unfold.
"My role can change a little bit, and that might mean playing a little bit more through the midfield and also playing in defence as well.
"The idea is to get as fit and strong as I can, and be prepared for whatever might come my way."
