He is understood to have told Manchester MP Graham Stringer:
I bet you didn't think things would actually get worse.
Jokingly, he added: "But I won't be appointing you as chairman of the campaign for me to return as leader."
And here comes the twist in our tale.
On Monday Miliband launched an all out defence of Corbyn, insisting that the new Labour leader has strengthened the party and saying he has what it takes to be prime minister.
I think [Labour] has a strength in depth in terms of our membership that we didn’t have before.
Jeremy Corbyn has doubled our membership and I’ve seen that myself in my own constituency as a constituency MP and I think this is quite important.
Asked whether Mr Corbyn could win the general election on BBC Radio 4's Today programme he said: "Of course. In the end that is a decision for the electorate, as I discovered to my cost. That is a decision the electorate has to make."