No. A verb in the passive voice does not need to identify the agent explicitly.
The man was murdered does not tell us who murdered the man, perhaps because the murderer is not known.
My wallet was brought to Lost and Found does not tell me who did it, perhaps because my wallet having been returned is important rather than who returned it.
However, the sentence
The patient was cured does not give the same information as
Doctor Smith cured the patient.
To give the same information as the latter sentence in the passive voice, you need
The patient was cured by Doctor Smith.
In short, when using the passive, you need not include the agent, but you always may do so.