The oldest player in an ageing squad, the 39-year-old lefthander's spot in the line-up is under scrutiny following his demotion down the batting order and as selectors look to the future.
With the promoted Travis Head taking his opening spot, Khawaja batted at number four in the third Adelaide test and number five in the fourth match in Melbourne.
While some reports in Australian media have suggested Sydney could be Khawaja's last international, McDonald said he had not had any conversations with him about his future.
"There's no indication at my end that he's calling it in Sydney," McDonald told reporters in Melbourne on Monday.
"But his performance in this calendar year has been good enough to warrant selection so I'd say he'll be there marking centre in Sydney."
Khawaja has not spoken publicly of his plans and returned home to Queensland for a break with family ahead of the Sydney test, which starts on January 4.
The 87-test veteran suffered back spasms during the first Ashes test in Perth and missed the second match in Brisbane.
He is averaging just over 30 with the bat in his three Ashes tests and scored 82 at Adelaide Oval to help Australia seal the series with two matches to spare.
However, he scored 29 and zero in his two innings in the fourth test in Melbourne as the hosts slumped to a four-wicket defeat.
While criticised for a soft dismissal in his second innings in Melbourne, Khawaja's numbers stack up better than several Australian batters this series, including opener Jake Weatherald, Marnus Labuschagne and all-rounder Cameron Green.
Australia lead the five-test series 3-1.
