The 35-year-old was a late omission from the 11 that began the four-day match east of London on Wednesday, replaced in the side by Peter Siddle as the tourists were sent in to bat.

Harris was sent for scans on the knee on Wednesday.

He had faced an uphill battle to unseat any of Mitchell Johnson, Mitchell Starc or Josh Hazlewood to reclaim a spot in any case after sitting out the West Indies series last month to be home for the birth of his son Carter.

His fitness issues almost certainly rule him out of the reckoning for the first Test in Cardiff, starting next Wednesday, and it is likely he will struggle to be in the mix for the second Test at Lord's that follows soon afterwards as well.

Head coach Darren Lehmann indicated after Australia's first tour match that selectors needed to be convinced that Harris could get through 20 overs in an innings before he could be considered.

He bowled 16 over and 14 overs respectively in the two innings against Kent, but now won't get the chance until Australia's next tour game against Derbyshire, which is scheduled between the second and third Tests.

Harris has a history of knee problems and said in Canterbury at the weekend that pain and clicking of the knee were going to be with him for the rest of his career.

"My knee's going to get sore here and there, that will always be the case," he said during the Kent match.

On this occasion, though, Australia announced that he hadn't recovered sufficiently from that outing, his first in nearly six months, to take the field against Essex.

Missing the start of the Ashes series shapes as a bitter blow for the veteran, far and away Australia's best bowler on the last tour of England in 2013 and having been set for this marquee series by Australia when they left him out of the World Cup squad.

He missed the first Test in 2013 at Trent Bridge when he was on the comeback trail from rotator cuff and Achilles injuries and was eager to play in all five Tests this time around.

Australian pace great Glenn McGrath forecast this week that Harris would probably not be chosen for Cardiff and instead saved for Lord's, where he was so outstanding in 2013.

"Ryan Harris hasn't played a great deal of cricket and they'll probably want to manage him throughout the series," McGrath said.

"So it might work out, there's always one guy fresh going into a Test."

Unfortunately, that scenario could even be in doubt now.