Thats just the point of prophecy - that prophecy was made long before Jesus was born. Unless he was God, and knew everything - how would he have known to ride into Jerusalem on a donkey in order to fulfill a prophecy? This isn't like a 500 year old prophet walking up to some guy and saying - i predict you will ride into jerusalem on a donkey, some guy says - ok ill do that - i must be the Messiah! Only if Jesus was God and the true messiah could he have known that prophecy 500 years after the fact.