Already Labour grandee and former Home Secretary David Blunkett has described Mr Miliband's speech at the TUC conference in Bournemouth this morning as a "critical juncture" in his leadership.
Thus far, there has been no apology from Mr Miliband over Falkirk despite the embarrassing U-turn of having accused Unite of "besmirching" the party, then having to admit publicly it had done nothing wrong.
While Len McCluskey, the Unite leader, has adopted a magnan-imous tone, urging people to move on from Falkirk, some delegates might not be so forgiving and give Mr Miliband a rough ride during his expected post-speech question and answer session.
His keynote address will be about embracing change with echoes of Tony Blair.
The Labour leader will tell conference about his proposal to shake up the way trade unionists pay affiliation fees to the party - from automatic to voluntary - saying: "We must have the courage to change.
"I respect those who worry about change. I understand but I disagree; it is the right thing to do. Change can happen. Change must happen. And I am absolutely determined that this change will happen. It is the only way to build a truly One Nation party so we can build a One Nation country."
Labour HQ, previewing the speech, insisted it would all be about building a "mass member-ship One Nation Labour Party rooted in the lives of working people and communities".
Yet in his speech Mr Miliband will accept that the vast majority of 3 million affiliated members, nine out of 10, might not sign up to becoming full members.
He will say: "This is an historic opportunity to begin bringing people back into the decisions which affect their lives. It means we could become a Labour party not of 200,000 people but 500,000 or many more.
"A party rooted in every kind of workplace in the country, a party rooted in every community in the country, a genuine living, breathing movement."
Seeking to repair bridges with the trade union movement, the Labour leader will attack David Cameron for showing contempt for ordinary trade union members.
"We have a Prime Minister...who doesn't just write you off but oozes contempt for you from every pore. What does he say about you? He says your members are a 'threat to our economy'. Back to the enemy within.
"Six and a half million people in Britain, who teach our children, who look after the sick, who care for the elderly, who build our homes, who keep our shops open morning, noon and night. They're not the enemy within. They're the people who make Britain what it is," Mr Miliband will insist.
While delegates generally welcomed the Labour leader's plan to ditch "exploitative" zero hours contracts if the party gained power, sources suggested there was anger among his lieutenants, some of whom were not told of the proposal in advance of it being trailed.
Meantime, in her keynote address Frances O'Grady, the TUC General Secretary, accused the Coalition of trying to divide Britain "Thatcher-style" between those in and out of work and between top rate taxpayers and everyone else.
She argued families were facing problems which "the Eton educated elite, with their serial holidays, hired help and inherited millions, simply haven't got a clue about".