Streeting, who admitted on the Today programme that he had not heard from the prime minister since last night's briefings, has spent the morning pledging loyalty to Sir Keir while denouncing his broader Downing Street operation.
That's a delicate balance to strike, especially when so many Labour MPs are eager to understand the extent to which Sir Keir himself will have been aware of or implicitly encouraged last night's briefings.
The key question this morning is whether the briefings have made Sir Keir more or less vulnerable.
Judging by the rancour of the messages I have received from various corners of the Labour Party, the definitive answer is: more vulnerable.
One senior Labour figure questioned why Sir Keir's allies had "legitimised what was a taboo" by publicly entertaining the prospect of a leadership challenge.
One minister said "this is pathetic," while another called the briefings "crazy".
An MP said the approach was "utterly unhinged and self-destructive," adding: "They're in the bunker shooting everyone who's outside the bunker guarding it. And poor oblivious Keir doesn't even realise he's in the bunker."
One cabinet minister expressed hope that the bitterness of this morning would eventually mean things calm down.
"These are the moments where you look over the cliff and don't like what you see at the bottom."
As expected, Conservative leader Kemi Badenoch made the most of Sir Keir's troubles at prime minister's questions.
He will be keen to avoid many more PMQs experiences like this.
And it is not immediately clear that his assertion that the briefings were "completely unacceptable" has soothed matters.
One minister messaged me afterwards to say that the prime minister's approach was "tone deaf".
And remember - it's only 14 days until the Budget in which Chancellor Rachel Reeves will make huge and potentially manifesto-busting judgment calls.
She now does so against this fevered backdrop.