Sticking in a ringer would be bad for the Democrats, but they have done it before and they have bounced back from it. In 1968 Humphrey was nominated, even though he hadn't run in a single primary. After Johnson's withdrawal and Kennedy's assassination McGovern was next in line according to the primary vote tally. The party didn't want McGovern so they just picked Humphrey. That's why the '68 convention drew massive protests.
If they chose either Biden or Kerry, each would have previously been vetted.