Can Islam explain the gap in time between Muhammad and the oldest available manuscripts of the New Testament?

If the idea of sending Quran through Muhammad was to replace the previously sent messages because they got corrupted later that means humans did not have the proper message to get guided between the time of the oldest available manuscripts of the New Testament and the Muhammad's time. There were a few centuries between these two and a few generations of people lived and died during this gap. Wasn't god serious about this issue? Is god serious about people having the correct message with them?