There is definitely a contingent of Sanderistas who were never Democrats and who will never vote for Clinton, very similar to the PUMAs who refused to vote for Obama. Clinton will never win them over.
But there is also a contingent of Sanders supporters who have been Democrats all their (mostly young) lives and who see returning to the New Deal/Great Society roots of the party as the only path for the future of the party. They’re not going to vote for Trump, but the concern is that they might go Green or not vote at all.
In the end, Clinton’s biggest enemy is probably apathy and lack of enthusiasm. The number of people who might stay home will massively dwarf any defectors to the Green party or even the Sanders concern trolls who will vote for Trump. The key is to inspire the apathetic and unenthusiastic. And I’m not sure how she’s going to do that.1
The margin between Obama and McCain was large enough that the PUMAs made no significant difference in 2008, but they definitely did exist and probably had an effect. And there are probably more hard core Sanderistas and the projected margin between Clinton and Trump is far slimmer.2
Pre-election polls from 2008 suggest that Obama might have done slightly worse (-1%) with Democrats than Kerry did in 2004 which might be the full extent of the PUMA effect.