Fremantle was trounced by St Kilda in Melbourne last week but was leading late in its last home game against Adelaide before losing that round three clash by 24 points.
But Roos said Fremantle's best was still very good.
"They're certainly playing good football in different parts of the game," Roos said on Thursday in Perth.
"They've got some exciting players, they've got some experienced players and they've got some very talented players.
"They've played better football than 0-4. The two teams they've played over here are both very good teams – Adelaide and the Bulldogs."
Roos said he had watched Matthew Pavlich closely this season and that Fremantle's under-pressure skipper is in reasonable form.
"Matthew's a star player of the competition and we rate him very highly," Roos said.
"We'll have some plans in place for him but we’re not sure where he's going to play.
"Like all those star players in teams that haven't won a game, they tend to cop most of the flak. But his form's been solid."
Roos' other major concern was Fremantle ruckman Aaron Sandilands, who will present Swan Darren Jolly with his biggest challenge so far this season.
The coach said Jolly and ruck partner Jesse White had been in reasonable form, but would need to be on guard with Sandilands.
"You put a lot of time and effort into the stoppages against Freo because you know that it is hard for your ruckman to get his hands on the ball," Roos said.
"Hopefully we can get our hands on the ball around the stoppages and stop their good players – Sandilands is one of them."
Brett Kirk (200 games) and Craig Bolton (150) will both reach milestones on Saturday.
Roos described the pair as the heart and soul of the Swans and said the club would note their achievements without going over the top.
The clash will follow the Anzac Day blockbuster between Essendon and Collingwood, but Roos said drawing on the emotion of the day wouldn't be too much of a focus.
"What we ask our players to do is far below what they were asked to do for the country," he said.
"So we touch on it, but out of respect we probably tend to focus more on the footy."
