Johnson, who will play his 150th game on Saturday, shapes as a key player against the Eagles, with his ability to play on tall and small opponents likely to be utilised.
Fremantle conceded an average of just 77 points a game last season (ranked No.2 in the AFL), holding West Coast to 6.12 (48) in the round 19 Derby.
The Eagles will have a full-strength forward line this time around, but Johnson said Fremantle would be ready.
"We love the big challenges (and) we want to play against these really good forwards," the 28-year-old said.
"We had a couple of good hitouts against them last season and I know as a defensive group we will have a game plan to match up on them and we'll be ready.
"I can't wait."
Johnson acknowledged that McPhee, who retired last November after a strong season, was a big loss, but said Fremantle had a flexible back six that could replace him.
Johnson played on x-factor forward Josh Hill and young tall Scott Lycett during Fremantle's NAB Cup clash against the Eagles and will be particularly valuable on Saturday.
"As a defensive group we can set up our match-ups on-field and we've got a number of players who can play a number of roles," he said.
"We know the key defenders will take the most dangerous and then we'll work around that.
"I can play up field or down back in deep. We'll wait and see [who my opponent is]."
Johnson described his career so far as a "great journey" and acknowledged there had been ups and downs, most notably his club suspension in 2010 and loss of form and confidence in 2011.
He said former Fremantle coach Gerard Neesham had been a mentor during his battle to re-establish himself at Fremantle, and new coach Ross Lyon had helped revive his career.
"When Ross came onboard he sat me down and we discussed where's the best position for myself and for the team and we both agreed in the backline," Johnson said.
"In the '06 season I played the majority in the backline there and that was one of my better years.
"I can go into games now knowing my role.
"There's been ups and downs, and I know when I first started I always wanted to play one AFL game, let alone 150.
"It'll be a great achievement for myself and my family."
Michael Johnson is a defender in NAB AFL Fantasy. He averaged 82.5 points in 2012. Register your team at our AFL Fantasy Hub.
Nathan Schmook is a reporter for AFL Media. Follow him on Twitter: @AFL_Nathan
