The Swans scored a comfortable win when the two teams met at the SCG in round 21, and the Bulldogs will be without star midfielder Adam Cooney and important defender Dale Morris.
Despite those losses, Roos said the Dogs' performance over the course of the season entitled them to pre-game favouritism for Saturday night's semi-final.
"Top four means you've clearly been better than the fifth, sixth, seventh, eighth teams in the competition," he said on Monday.
"We need to make up the gap, not the Bulldogs. Their best has been better than ours this year.
"We certainly still think they've got enough quality players to do some damage in the finals and … we've got to play as well as we [did] for three quarters yesterday. If we play like we did in the third quarter, we'll really struggle."
The Bulldogs will also be motivated by prospect of extending captain Brad Johnson's career by at least another week.
The 362-game veteran announced his decision to retire at the end of this year's finals series, meaning Saturday night could be his last game if the Dogs lose.
Johnson was far from his best against Collingwood and finished the game on the bench with a sore achilles, but Roos said he remained a significant threat for Rodney Eade's side.
"He's probably a bit like 'Kirky' (Brett Kirk). People were criticising 'Kirky' in the middle of the year … champion players find a way, and he's a champion player," he said.
"He'll bounce back, and he's one that we'll really have to watch this Saturday night.
"You can tell Rocket's got a lot of confidence in Brad Johnson. If I was Rocket, I'd have a lot of confidence in him as well because he's a genuine champion of the game and they always find a way to get things done."
The MCG has been something of a graveyard for the Swans in recent years. Ross' side has lost nine of its last 10 matches at the venue, and was thrashed by 73 points by Melbourne in round 17, its last visit to the ground.
However, Roos dismissed the suggestion that the players had suffered mental scarring from the heavy defeat.
"I certainly hope not," he said with a laugh. "They might as well not turn up if they do."
