On the back of the 45-point loss to Collingwood at Telstra Dome on Saturday night, Roos conceded injuries had damaged the Swans' recent form, which has produced just two wins from their past eight matches.
"We've had our best players not playing – Barry Hall, Mick O'Loughlin, [Tadhg] Kennelly at times and [Nick] Malceski, who was second in our best and fairest last year – so there's no doubt it has a significant effect. There's no question whatsoever.
"In '05, we didn't have a lot of injuries and in '06 the West Coast Eagles were the same and last year Geelong. So in order to be a very good team – top one or two or [winning] the premiership – you do rely on your best players.
"This year has been a lot more challenging. It's tough when blokes aren't playing, but that's just the way footy is."
Despite the club's personnel issues, Roos said the first half against the Pies, when the Swans kicked four goals to 11, was unacceptable.
"You'd think we'd come ready to play, but obviously at the moment the effort is just not there," Roos said.
"It's disappointing, because it's a big game and you're playing for higher stakes, but the guys are incapable of coming with the required intensity to compete at AFL football the way it's played now."
Asked why the Magpies had the wood over the Swans – they notched up their sixth win in a row – Roos said: "When you've [won a] premiership in 2005 and we're back-to-back grand fFinalists, teams generally benchmark against the better teams and over that period of two years, we were obviously one of better teams in the competition.
"In a way it's a compliment to us, because they respect us as a footy team and they get themselves up to play us and they play at a very high level when they play against us."
Although there is a possibility the Swans will play the Pies again in the first week of the finals, Roos was not drawn into considering another match-up.
"I think it's more about Brisbane next week to be perfectly frank. I can't be too worried about who we're playing in two weeks' time," Roos said.
"We've got to make sure we finish off with a win next week to at least give us some sort of confidence going into the finals. That's the priority for us – to get a win against Brisbane at the SCG next Saturday night."
