It was clear from the prime minister’s very personal and handwritten letter to his fallen deputy Angela Rayner, that this was a resignation he did not want and deeply regretted.
And so he should: Ms Rayner’s departure from his government is a huge loss.
She is a powerhouse within the Labour Party and a figurehead of the left, who brought Sir Keir’s project some legitimacy from quarters of the Labour movement minded to give him short shrift.
Describing her as a “true friend” and the embodiment of the sort of social mobility he would want to leave as his legacy in office, it was clear that the prime minister both mourned her departure and wanted to keep his formidable and politically astute former deputy onside.
Because her departure does create difficulties. She is badly-wounded now, but could be a beacon on the backbenches should she seek to rebuild her political career and climb the ladder once more.
Ms Rayner is a politician deeply admired in pockets of the party and a product of the trade union movement.
Politics latest: Cabinet reshuffle after Rayner quits
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There is no sense that she would be disloyal to Sir Keir but she is a figurehead whether she wants it or not.
The manner of her departure – forced out because she underpaid her tax and in doing so failed to uphold the highest standards in public office, is also difficult.
Sir Keir’s rivals will be armed with plenty of ammunition this Budget should the chancellor choose to raise property taxes even as a leading Labour figure underpaid theirs.
There is also the matter of the deputy Labour leadership race: this is an elected post that Ms Rayner quit on Friday and already there is a battle emerging between the party’s left and centre right over who should succeed her.
As the saying goes, “divided parties don’t win elections” and Labour’s divisions are now about to be put on full display in a very public race to replace Ms Rayner at the top of the Labour Party.
But for all these difficulties, Sir Keir on Friday sought to turn a crisis into a catalyst for change by completely overhauling his top team in a reshuffle that even serving cabinet ministers had no clue was coming.
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This is a prime minister that is trying to reinvigorate a flagging government and reset a stuttering Number 10.
It was decisive and defiant.
For months he has been under fire for being too careful and cautious, tentative and timid.
But today he has rolled the dice and changed the top team in an effort to reset the government and get back to winning ways.
The biggest changes reserved for the two of the biggest priorities: a new home secretary in Shabana Mahmood tasked with tackling the small boats crisis and a new super ministry, headed up by Sir Keir’s close ally Pat McFadden, to tackle the burgeoning welfare bill and get people back to work.
One senior figure told me on Friday night that the prime minister had been thinking hard over the summer about having a “kitchen cabinet” – a phrase which alludes to a band of trusted advisors – to give him more advice.
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In this reshuffle you can see some of the fruits of that, with his leadership campaign manager and close friend David Lammy becoming deputy prime minister and Jonathan Reynolds moving to chief whip – a critical role for a prime minister struggling with his backbenches – and Mr McFadden tasked with another key job.
“What he needs are politicians he feels comfortable with,” said one figure. “This is about the PM reaching for the right kind of support and a team he trusts and can rely on”.
Ms Rayner is no longer around that table but she will remain a powerful force in the Labour Party and British politics as we watch what she does next from the backbenches.
Sir Keir will hope his fresh top table can restore some of his power too. It is hard to see now how this prime minister gets back to his winning ways, but this reshuffle at least showed an appetite to try.
