Shaykh of shias Jamalutdin al-Hilli in his “al-Alfayn” (p 23)  said:

الإمامة لطف عام ، والنبوة لطف خاص لامكان خلق الزمان من نبي حي ، بخلاف الإمام لما سيأتي ، وإنكار اللطف العام شر من إنكار اللطف الخاص ، وإلى هذا أشار الصادق عليه السلام بقوله عن منكر الإمامة أصلا ورأسا وهو شرهم

Imama is general favour (from Allah) and prophecy is especial favour (from Allah), it is possible for time to be without alive messenger, but it is not possible for it to be without alive Imam, as it would come (1). And rejection of general favour WORSE than rejection of especial favour, and to that points Sadiq (alaihi salam) by saying regarding rejection of Imama (is) foundation and head and it is worse of them.

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1) Obviously in the printed version is an error. And we corrected this part  it is possible for time to be without alive messenger, but it is not possible for it to be without alive Imam, as it would come, in accordance to the book of Ali al-Bahrani “Manar al-Hadi fi Nass ala Imamati Isna Ashar” p 52.