The scrum-half - widely regarded as the world's best player - aggravated a right knee injury in Australia's 28-26 win over New Zealand in Auckland last Friday. "I'll see my Newcastle Knights doctor for scans and the results will determine my fate," Johns said.
The Kangaroos fly to London on Thursday for their two Tests against Great Britain. Johns, 31, underwent a total reconstruction on his right knee last season and was ruled out for much of 2004 as a result.
"I'm hoping it will be nothing serious and I can be OK for the rest of the tour," Johns, who has played in 20 Tests for Australia, said. "The knee is giving me grief and has hampered me in the two Tests against New Zealand. I couldn't run properly on Friday and it's not the way I want to play.
"I haven't been happy with my performances and that's what I have to sort out within myself. If I go to England, I want to contribute to the team. I don't want to play and be half-fit and feel as though I could be doing more to lift the side," he said.
Australia play the first match of their tour at Wigan on 5 November.
The new Great Britain vice-captain, Brian Carney, insists the Lions have the ability to win the Tri-Nations Series whether Johns plays or not. Despite being without the likes of Paul Sculthorpe, Kris Radlinski, Sean Long and last year's captain Andy Farrell, Carney is confident Great Britain can cope.
"They are definitely a loss because they are outstanding players and immensely talented," he said. "But sometimes when you are missing your big players, somebody steps up and shows what they can do." Great Britain will go into Saturday's opening match against New Zealand at Loftus Road as underdogs after the Kiwis' impressive performances in the opening two games of the series against Australia.
But Carney is relishing his new role of vice-captain and said he was surprised to have been given the job. "It is a great honour and a great privilege, but I was shocked when Brian Noble rang and asked me to take the position," he said.
"I think people have got to understand that the position has been given to me for a lot of different reasons that people don't often see.
"It's not because I'm a master tactician on the field or anything close to that. The people who excel at that, your halves and your ball-players, will still maintain that job. I guess Brian just felt I could provide an efficient link between the coaching staff and the playing staff. That's the role I'm assuming - nothing else."
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