Why did Eliezer pray? Did he not trust in Avraham's merit?
Ohr ha'Chayim: He trusted that Hashem would send an angel in the merit of Avraham, but was unsure if he is worthy that the matter succeed through himself.
What is Hakrei?
Malbim: 'Koros' are unexpected matters. Yakov's sons told him "ha'Koros Osam" (42:29), for it is abnormal that they should be called spies. Eliezer requested special Hashgachah, that it will occur today, now, like he says.
Ha'Emek Davar: Eliezer did not know whether or not the proper girl is in this area. He requested Hashgachah that if she is, she will come to draw and answer like he said, and if not, no girl will do so.