Cook's 32 on Thursday was his best knock so far in Australia
Former Australia captain Ian Chappell believes that England's decision to put faith in Alastair Cook for the winter's Ashes series is "a hell of a gamble".
Aside from a score of 110 in the Oval Test against Pakistan, the left-handed opening batsman, 25, has been in poor form for England since May.
"It wouldn't surprise me one little bit if Cook fails in the Tests.
"I'm absolutely staggered they don't have a third opener to move into the team if Cook fails," said Chappell.
"He hasn't started the series too well in the first two warm-up games. He has got a lot of flaws in his technique and they are going to be exacerbated under Australian conditions."
Chappell, speaking exclusively to BBC Sport, highlighted England's "Achilles heel" as their top-order batting, and feels Cook will have trouble coping with the new ball in the Test matches.
On Thursday against South Australia, he pushed outside off-stump uncertainly to be caught behind after making 32, a slim improvement on scores of five and nine in the first match in Perth.
I'm absolutely staggered they didn't have a third opener to move into the team if Cook fails
His overall form since the first Test against Bangladesh at Lord's has been discouraging. The century at The Oval aside, his best score in nine other Test innings since the start of the last English summer is 29.
Coach Andy Flower said on Tuesday he had "no concerns" about Cook, adding "I think he's going to do well."
And team-mate Paul Collingwood said at the close of play in Adelaide on Thursday: "I thought he looked in good nick today. He was moving really well, his shot selection was pretty good. It's just a shame that he got out like that."
Chappell said England were probably right to pick Cook in the Ashes squad, but felt the absence of a stand-by opener left team management relying too heavily on him to come good.
"I wouldn't have picked the side that way with Cook's name in ink, it would have been pencilled in, and the fact he's struggled in the first two games doesn't bode well.
"It means you are going to have to make major changes to the batting order if Cook continues to fail.
"If England were to win the first Test and Cook fails, then maybe that's OK. But if they lose the match and Cook fails then the question falls to the selectors: 'How far are you prepared to go?'
"They may have to send Jonathan Trott to the top of the order, and suddenly you go from having a settled side to one that's anything but."
Trott, England's most in-form batsman last summer, is the only other top six player not to hit a half-century yet on tour. The Warwickshire star has settled at number three and has an average of 55.00 in 13 Tests.
He has opened on one occasion, against Bangladesh in Chittagong in March, when he scored 64 and 19.
England's back-up performance programme squad recently lost its only specialist opener with Test experience, Michael Carberry, to illness. Their remaining openers are Yorkshire's Adam Lyth and Hampshire's Jimmy Adams.
Ian Chappell is part of the team bringing you live Ashes commentary on Test Match Special on BBC 5 live sports extra, Radio 4 LW and online (UK users only) - series starts 25 November 2010.
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