Detach a process from Sh (not Bash) or “disown” for Sh (not Bash)

Consider a shell script executed by Sh, not Bash (and I can't change it, and can't use a shebang, it's ignored). The & operator works, but disown $! does not and makes Sh complains “*disown: not found*”. How do I detach a background process from specifically Sh? I mean, doing from Sh the same as disown do from Bash.