The Sadguru is Man-God (i.e., man becoming God) and had to pass through the process of evolution and involution, whereas the Avatar is God-Man; that is, God directly becomes man without passing through the process of evolution and involution.
The five Sadgurus (Qutubs, Perfect Masters) effect the advent of the Avatar (Rasool, Christ, Buddha) on earth and therefore the advent of the first Avatar on earth was not possible without there first being the five Sadgurus to effect that coming. Consequently in the beginning the five Perfect Masters became realized first and there took place the first advent of the Avatar on earth.
Whether there have been twenty-six Avatars since Adam, or one lakh and twenty-four thousands of Prophets as is sometimes claimed, or whether Jesus Christ was the last and only Messiah or Muhammad the last Prophet is all immaterial and insignificant when eternity and Reality are under consideration. It matters very little to dispute whether there have been ten or twenty-six or a million Avatars.
The truth is that the Avatar is always one and the same, and that the five Sadgurus bring about the advent of the Avatar on earth. This has been going on cycle after cycle, and millions of such cycles must have passed by and will continue to pass by without affecting eternity in the least.
–GOD SPEAKS, p. 249 (Meher Mownavani Publications)