The dilemma facing the YPG generals in Afrin now is to either send away the civilians from the densely-populated city and fight to the death in the forlorn hope that help might arrive as was the case in Kobani, or to preserve their lightly-armed forces by pulling them out to Manbij where the Turkish-jihadi barbarians intend to head next.
The YPG cannot in good conscience wage a prolonged battle in the city if civilians refuse to leave their homes as it seems to be the case. The Turkish-jihadi barbarians have the firepower and the willingness to level Afrin city and to kill tens of thousands of civilians in order to capture it. The YPG may have to consider withdrawing their forces to fight another day.
The third possibility is to invite Syrian Arab Army proper into Afrin. In effect that would amount to surrendering the YPG forces to the Assad regime. Even if the YPG surrenders to the regime, the bulk of the best-armed regime forces are fighting in Eastern Ghouta at the moment; the regime forces in Aleppo cannot resist Turkish jihadi invaders if the attacks continue.
There is the fourth possibility: the YPG forces do not surrender to the regime but withdraw from the city to only be replaced by the regime forces. The question is whether this will be sufficient for Russia to order a halt to Turkish advance, or whether the momentum to take Afrin is too great for this possibility to have any effect.
If Afrin city was to be evacuated of all of its civilians, that would have started already with the fall of Jinderes. Since there does not appear to be any civilian movement out of the city, the YPG may well have to withdraw their forces to Manbij where the USA has promised to protect them.