Well, there is still time for a relatively unknown backbencher to come forward. I think something similar happened the last time Burnham ran for the leadership.
Support for Corbyn, who entered the race as a dark horse candidate,[2] and the release of opinion polls which showed him leading the race, led to high-profile interventions by a number of prominent Labour figures including Gordon Brown,[3] Tony Blair,[4] Jack Straw,[5] David Miliband,[6] and Alastair Campbell, among others,[7] many of whom argued that Corbyn’s election as leader would leave the party unelectable.
Despite these interventions, Corbyn was elected in the first round receiving 59.5% of the votes, winning in all three sections of the ballot. Less than a year later, a leadership challenge saw another leadership election, where Corbyn again won, with an increased share of the vote.
Well, there is still time for a relatively unknown backbencher to come forward. I think something similar happened the last time Burnham ran for the leadership.
For the unaware:
https://en.wikipedia.org/wiki/2015_Labour_Party_leadership_election_(UK)