The process of turning Silas back into a human was fairly dangerous. After all, his head had to first be reattached to his body before he could be injected with the stronger serum.
If he were to be injected with the serum before his body was attached to his head, those two separate parts would regain their human form, thus making it impossible to fuse those two parts back together without the regenerative strength of a vampiric form.
Unfortunately, there was a risk that came with the process. For one thing, Silas would lose his current human-like thinking and revert back to the demonic, vampiric persona. He would attack anyone on sight, and if not properly controlled, could result in injuries, deaths, or worse― more vampires.
Hence, there was only one person who could safely carry out this process.
