If Starmer has a brain he'll put up an absolute no-mark, spend not one penny, and let Corbyn walk it - which he probably will anyway for the reasons
Rob Ray suggests.
This allows him to spend resources where he might actually win, and to slap a lifetime ban on both Corbyn for standing against a Lab candidate, as well as anyone else who so much as likes one of his campaign tweets.
Corbyn gets to be an indy MP, Starmer still gets a Lab govt, and he gets to clear out Labour of any remaining Corbynites.
It's an interesting problem for the Lab left - Corbyn will certainly get to sit as a left figurehead in parliament, but it will almost certainly mean that his constituency within Labour gets expelled, which looks like a somewhat Pyrrhic victory...