Sydney coach Paul Roos isn't getting too comfy in the AFL penthouse and is expecting to lose star defender Craig Bolton for up to six weeks.

Four successive wins following an opening-round loss to St Kilda have lifted Sydney to top spot on percentage prior to Monday's final game of the round.

It represents the Swans' best start for 12 years, but Roos recognises they face a testing month ahead.

After knocking off several of the early-season stragglers, the Swans face a series of fixtures against some of the other pacesetters.

They play Brisbane at the SCG this weekend followed by Geelong at Skilled Stadium, Western Bulldogs at Manuka Oval and Fremantle at the SCG.

"It's a long way to go yet, if it was round 22, I'd be happy," Roos said on Monday.

The Swans boast the second stingiest defence in the competition behind St Kilda, conceding just an average of 70 points through their first five games.

However, they stand to lose co-captain and defensive lynchpin Bolton for between three and six weeks following an achilles injury he suffered in last Saturday's win over West Coast.

"He will get a scan on his Achilles tomorrow, he's struggling to walk, I would say he's no chance to play this week," Roos said.

"We will get a better idea of how he is, it may be a long term injury."

While Bolton won't be around to play his former side this weekend, Roos is happy to entrust the key defensive jobs on key Brisbane forwards Brendan Fevola and Jonathan Brown to Lewis Roberts-Thomson and Heath Grundy respectively.

The two Lions forwards are among the leading early season goal kickers, with Brown topping the chart with 20 and Fevola joint fourth on 17 with Sydney's former Brisbane spearhead Daniel Bradshaw.

Roos said he believed Fevola had adjusted well to the Brisbane forward setup.

"He backs up and kicks goals when Brown is a bit quiet and then when Brown is on fire, as he has been a couple of times, we've seen him take his man out and letting Brown lead back in, so I think they are working really well together," Roos said.

Roos wasn't reading much into Brisbane's surprising first loss of the year to Melbourne.

"I don't think you can take too much out of it from a Brisbane point of view because they have had a tough run and maybe they were due for a downer and they will bounce back this week," Roos said.

The Lions, along with the Swans, were one of the main players during last year's trade week and Roos said he believed most of Brisbane's recruits had done well.

Roos doubted whether recruit Ben McGlynn, who has been a revelation for the Swans this season, would recover from his quadriceps injury suffered in round four in time to face Brisbane.

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