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The union leader told Politico that a Labour victory on 8 June would be "extraordinary" given the state of the party and criticism of it in the media.
He suggested winning 200 seats - nearly 30 fewer than in 2015 - would be a "successful" result for Mr Corbyn.
It comes as the Tories said there was a £58bn "black hole" in Labour's plans to nationalise two key industries.
Labour's manifesto - published on Tuesday - did not set out how plans to nationalise the national grid and the water industry would be funded.
The Conservatives said bringing them back into public ownership, which would require a future Labour government to compensate existing shareholders, would add £14bn to the national debt in one year alone.
But Labour, which has also pledged to nationalise the railways and Royal Mail, said it could be done at no net expense to the public purse and that consumers would see their bills come down by as much as £220 a year.
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Mr McCluskey's intervention came hours after Mr Corbyn launched his party's manifesto, which the Labour leader described as a "programme of hope" and which included billions for schools and the NHS and an expansion of free childcare.
The leader of the UK's biggest union said he was supportive of the manifesto but was not "optimistic" about Labour's chance on 8 June, given the hostility which he claimed it faced in large sections of the media.
"In terms of the imagery of Jeremy, that's a huge task," Politico quoted him as saying.
"He's got now just under four weeks to try to see if you can break through that image and it's going to be a very, very difficult task...whether that breakthrough can happen, we'll wait and see.
"I'm not optimistic, but we'll wait and see."
He added: "People like me are always optimistic… things can happen. But I don't see Labour winning."
He went on to suggest that if Labour emerged with 200 seats - which would be about 30 fewer than Ed Miliband secured in 2015 - it would represent a "successful campaign" given the circumstances it found itself in. Should than happen, it would be Labour's worst result since 1935.
"I believe that if Labour can hold on to 200 seats or so it will be a successful campaign," he said.
"It will mean that Theresa May will have had an election, will have increased her majority but not dramatically."
Mr Corbyn was quoted as saying earlier in the campaign that he would stay on whatever the result of the election, citing the mandate he has from his leadership victories in 2015 and 2016.
Both Gordon Brown and Ed Miliband resigned after leading Labour to defeats in 2010 and 2015 but, amid speculation that jockeying for position has already begun, there have been suggestions that Mr Corbyn could stay on if he equals the 30.4% vote share that Ed Miliband got in 2015.
Labour is defending its plans to exert state control over key utilities amid uncertainty over what the final price tag will be and how it will be paid for.
It said it would not put an "arbitrary figure" on how much it would cost to nationalise England's nine water companies and the National Grid, saying the compensation due to existing shareholders would depend on a range of factors and would ultimately be decided by Parliament.
But it said it would be achieved by exchanging government bonds for shares in the relevant companies, and since profits currently used to pay dividends would be used instead to pay interest on those bonds, they would be "no net cost" to the exchequer.
The Conservatives criticised "the premise that nationalising the water market and energy grid don't add to borrowing", saying the result would inevitably be higher taxes or rising bills.
"Jeremy Corbyn can't deliver any of this," a Tory spokesman said.
"These are made up numbers, based on a shambolic manifesto with a £58bn black hole at its heart. While his figures are a fantasy, it is ordinary working families who will pay."
Labour, which has said it would return the railways to public ownership as franchises expire, has also made a series of multi billion-pound spending commitments on health, education
It insists they are fully costed and will be fully covered by tax rises, including increasing corporation tax from 19% to 26% and raising the top rate of tax for earnings above £80,000 to 45p and to 50p for income over £123,000.
But the Institute for Fiscal Studies has said it was "genuinely uncertain" whether increases to income tax would raise the £6.4bn Labour has earmarked, adding that they represented a "big increase" for high earners.
Labour has also said it would return the railways to public ownership as franchises expire, or in some cases using franchise reviews or break clauses.