The Waratahs have suffered a blow with the news that injury-prone playmaker Berrick Barnes will undergo thumb surgery, ruling him out for six weeks.

Barnes had just recovered from a knee injury and made his return to the Waratahs side in their loss to the Cheetahs last week, but lasted just 17 minutes before injuring his thumb in a tackle.

There was further bad injury news for Waratahs coach Michael Cheika, with Wallaby forwards Tatafu Polota-Nau (hamstring) and Wycliff Palu (ankle) both expected to miss the match against the Blues in Sydney on Sunday.

Despite their lacklustre start to the season, Waratahs assistant coach Alan Gaffney insists that his team can catch the unbeaten Brumbies who currently top the Australian conference.

"The Brumbies can be caught, nothing is ever sure in rugby. We will be chasing them," he told AAP.

Gaffney explained that many of the players are still adapting to the new approach that they have adopted this season.

"Some players have been regimented in the way they've played over a period of time," Gaffney said.

"But now they are being given a lot of freedom. As yet, we haven't totally seized that freedom, but it will happen."