Then we don't have a crusade.
Which may actually be
better for the Crusader States in the long run. OTL their chronic problem was understaffing; they had plenty of institutional 'support' (money) coming in from Europe, but not very many men on the ground to man the fortresses.
Crusaders coming from Europe rarely solved this problem: the crusaders often had their own ambitions which were completely at-odds with the actual foreign policy of the Crusader States; the crusaders were often completely ignorant of or disapproving of the necessary compromises their Outremer counterparts had made to secure their rule, and they almost always went home immediately afterwards. So any manpower gain would be temporary and not actually solve the long-term problems caused by the lack of actual Latin settlers in the Levant.
However, it's hard to prevent Edessa from being taken without giving the Crusader States some extra edge; they need to make up that staffing shortfall somewhere, ideally from somewhere other than 'seasonal' help from the Europeans, or else there's no reason why Zengi
wouldn't try to take Edessa (or succeed, for that matter).