Kangaroos legends Steve Mortimer and Brett Kenny believe Trent Barrett must be selected in the halves should a chronic knee injury rule champion halfback Andrew Johns out of the Tri Nations.

Johns aggravated his right knee in Australia's 28-26 victory over New Zealand in Auckland on Friday night and is in serious doubt for the remainder of the series against the Kiwis and Great Britain.

Johns was inspected by Newcastle club doctor Neil Halpin and will undergo MRI scans on Monday morning. Halpin will then consult with orthopaedic surgeon Michael Johnson and Kangaroos doctor Hugh Hazard before they make a final ruling.

"He's got a sore knee alright, it's his right knee, the one he had an anterior cruciate reconstruction on," Halpin said.

"The reconstruction is intact, there's no problem with that, but what he seems to have is some femoral pain, which is common after cruciate reconstructions.

"Joey says it's very sore, he's certainly got nothing to prove in terms of toughness and playing with pain.

"Andrew is not over-confident that he'll be right, so we'll just have to see what the MRI shows and speak to the other doctors involved.

"The MRI will show if there is any other damage going on, but patello femoral pain is ... fairly common after cruciate reconstructions, he can feel the grating in his knee.

"It's pretty sore, there might be more going on that that."

The Australians have three options at halfback should they need to replace Johns - Penrith's Craig Gower, Clive Churchill medallist Scott Prince and Barrett, who normally plays at five-eighth for St George Illawarra but has filled in successfully at No.7 at representative level.

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