The binary "man" (/usr/bin/man), is in SUNWdoc package, so you just need to reinstall this package usualy from Solaris CD Software 1 of 2, just mount the CD and go to dir "Product" and add wanted package from list, i.e in Solaris 8 (Release 04/01), with CDrom mounted on /cdrom:
#cd /cdrom/sol_8_401_ia/Solaris_8/Product
#pkgadd -d .
...
RETURN> for more choices, <CTRL-D> to stop display:
71 SUNWdoc Documentation Tools
...
on this release is package number 71:
Select package(s) you wish to process (or 'all' to process
all packages). (default: all) [?,??,q]: 71
...
if you don't want install from CD you also can "export" SUNWdoc package from another machine with same Solaris version, and cp and install in target machine.
Hope it helps,
Carlos Almeida,