The given /etc/init.d script specifies an interpreter in its shebang located under /usr.
It indicates that the init script may be using a non-essential interpreter. Since init scripts are configuration files, they may be left on the system after their package has been removed but not purged. At that point, the package dependencies are not guaranteed to exist and the interpreter may therefore not be available.
It's generally best to write init scripts using /bin/sh or /bin/bash where possible, since they are guaranteed to always be available.