So, there will be a primary of the left after all. Or rather, a primary of the "left." Jean-Luc Mélenchon, who excluded himself in advance, will not be part of it. Neither will the PCF or the Trotskyists. This one will be limited to the "governmental left," meaning the PS, certain Greens (presumably excluding Duflot), and the PRG. Hollande gave his approval, probably because he thinks he will win, even though a recent poll showed that only 7 percent of the French want to see him serve a second term. But he could hardly declare himself the "natural candidate" of the left under these circumstances. So he chose the next best alternative.
But it's by no means clear that he can win even this contest, of his own choosing, designed by his own party. Hollande was a protégé of Mitterrand, who created the party of Epinay, the party that embraced the PCF and chose to have no (overt) enemies to its left. As I have said before, Hollande has been the (unintentional?) gravedigger of that party. But what he's left with is hardly a party at all, just a collection of ambitions. Some of the them are large: Arnaud Montebourg, for example. Others are small, e.g. the senator Marie-France Lienemann, the only announced candidate thus far. But who knows how many others will dip their toes to take the temperature of the waters? There could be many, and each one will take votes from Hollande--if he runs, which of course he may decide not to do if things look really dire.
One thing is sure: the victor, whoever he or she might be, will not be assured of a place in the second round of the presidential. This is Hollande's gift to the part after 4 years in office.