McGrath left the team here in north Queensland for Sydney where he will consult a specialist to determine whether he needs an operation on a troublesome injury.

Team physiotherapist Errol Alcott said the injury was caused by a loose fragment of bone near the back of the ankle joint.

He said an operation would keep McGrath out of action for about five weeks.

With only 10 weeks before the start of the series against Zimbabwe in October, it was decided it was an opportune time to assess the damage.

McGrath will miss the one dayers scheduled for Cairns on Saturday and Sunday and Darwin on August 6.

It is the third disruption McGrath has had this year after he missed the fifth Ashes Test against England with a back injury and two Tests against the West Indies when he flew home from the Caribbean to be with his wife, Jane, after she was diagnosed with cancer.

McGrath acknowledged the injury breaks could have contributed to his current state of fitness.

"I've always said that if I keep playing the body gets used to it and I'm fine but getting back into it can sometimes be a worry," McGrath said.

"The back injury during the Ashes was a bit strange but we've got on top of that now and then with what Jane and I went through at start of the West Indies you can't plan for that and then this now..."

McGrath and Alcott believe the injury stems from his high school basketball days, with a severe sprain pulling some bone off the ankle which has since floated around in the joint.

"It's his left ankle and the pain has increased over the last few days," Alcott said.

"It was particularly uncomfortable in the last two days of the (second) Test. The joint's a bit angry at the moment.

"There's a window now of two months so we thought we'd send him home to an ankle specialist and see what happens from there."

McGrath said he had put up with varying levels of pain for most of his career but generally coped by taping the ankle.

"It's been an ongoing thing but hopefully now we get on top of it while we can and be ready for when the season starts properly."

West Australian fast bowler Brad Williams was later named as McGrath's replacement and he will join the team on Friday.