I understand that Allah has never left this universe without a Hujjat. Who was doing this duty between the time of Prophet Issa AS and Prophet Mohammad SAWS for several hundred years?
The Hujjat is must in every time, either publicly known or hidden from public for reasons, as Ameerul Mo'mineen (AS) said (Nahjul Balaghah, 497).
After Prophet Easa (AS) the Hujjat was Sham'oon Ibn Hammon al-Safa, then Yahya ibn Zakaria, the Munthir, then Sulaimah, then Burdah, then Muhammad (SAWA).(Amaali al-Tousi, 443)
In al-Kaafi 1:177 you can find many Hadeeths in this regard.
Khalib ibn Sinan has been mentioned as a prophet fifty years before Prophet Muhammad ( Rawdhat al-Kaafi 342).
Abu Talib (AS) was from deputies (AWSIYAA') after the prophets.
Ameerul Mo'mineen Ali (AS) was asked: Who was the last Wasi ( Deputy) before Muhammad? He replied: My father. Al-Ghadeer 7:389 from Dhiaul Alameen by al-Fotouni.
Wassalam.