Restore man user

There is a utility called update-passwd that appears to do this safely for you:

NAME
       update-passwd - safely update /etc/passwd, /etc/shadow and /etc/group

SYNOPSIS
       update-passwd [options]

DESCRIPTION
       update-passwd   handles   updates   of   /etc/passwd,  /etc/shadow  and
       /etc/group on running Debian systems.  It compares the current files to
       master  copies, distributed in the base-passwd package, and updates all
       entries in the global system range (that is, 0–99).

First check the current entry

$ getent passwd man
man:x:6:12:man:/var/cache/man:/bin/sh

Now let's delete it

$ sudo deluser man
[sudo] password for steeldriver: 
Removing user `man' ...
Warning: group `man' has no more members.
Done.

Check it's really gone

$ getent passwd man
$ 

Then

$ sudo update-passwd --dry-run
Adding group "man" (12)
Adding user "man" (6)
Would commit 2 changes

OK let's do it

$ sudo update-passwd
2 changes have been made, rewriting files

Now confirm the restored password file entry

$ getent passwd man
man:*:6:12:man:/var/cache/man:/bin/sh