A Fabian Society report said the party was "too weak" to take power without support from other parties.
Ms Stevens said Labour could win if it was "bold and credible and radical enough" in its manifesto and policies.
Welsh Labour vice-chair Mike Payne said UK leader Jeremy Corbyn had "failed to make the impact" allies predicted, but Carwyn Jones in Wales was trusted more.
The report claimed that Labour was set to win fewer than 200 seats whether an election took place in 2017 or 2020, and even if its poll ratings improved would still have "no chance" of winning a majority.
Ms Stevens, the MP for Cardiff Central, rejected the study's claim that Labour could only take power at Westminster with the backing of other parties.
"If Labour is bold and credible and radical enough in the manifesto and the policies it puts to the country, then Labour can beat the Tories by itself," she said.
"Obviously the polls at the moment aren't suggesting that, but if we don't have an election until 2020 who knows what's going to happen between now and then.
"Look at what happened last year. Politics is very unpredictable at the moment."
Ms Stevens dismissed the idea of a "progressive alliance" of parties that could include Labour.
"Minor parties would want to work with Labour because they think that Labour won't stand candidates in very few areas where there's a possibility a minor party might win a seat," she said.
"When it comes to the other way around - minor parties not standing candidates so Labour can stand and win - those minor parties don't stick to their side of the bargain.
"As someone who beat a Lib Dem sitting MP in a marginal seat in the general election in 2015 I've no interest at all, and neither have many of my colleagues, in any sort of alliances with the Lib Dems because we saw what they did in coalition government with the Tories.
"[Lib Dem leader] Tim Farron is on record as saying they would go into government with the Tories again.
"So I think it's a complete red herring."
In Wales, Labour held onto power after the 2016 assembly election by including sole Lib Dem AM Kirsty Williams in the cabinet and striking a deal with Plaid Cymru to support Carwyn Jones as first minister.
"The Welsh dimension is slightly different," said Mr Payne, who becomes Welsh Labour chairman in March.
Referring to Mr Jones, he added: "The Welsh Labour brand has a leader that is trusted more than Jeremy Corbyn is trusted.
"At a UK level Jeremy Corbyn has failed to make the impact people were suggesting he would.
"I personally don't see us making any headway while Jeremy Corbyn is leader of the party.
"Even Jeremy Corbyn's staunchest allies are starting to doubt he can win a majority."
On Sunday, Unite union leader Len McCluskey suggested the Labour leader could step down if the party's poll ratings fail to improve.
But he later tweeted his "full support", describing Mr Corbyn as a "genuine, decent man fighting for a fairer Britain".
Shadow Welsh Secretary Jo Stevens has rejected as "utter nonsense" the idea Labour cannot win a general election.