Ricki obviously thinks he's a better bet if he let Rees go to make way for Mulligan.
Here's an extract from a Port Vale site which implies Mulligan is struggling to get a start - unless he's been injured.
"Meanwhile, Vale's Kiwi right-back Dave Mulligan is a target for New Zealand side Wellington Phoenix. The club, who play in Australia's A League, are managed by Ricki Herbert, who also manages Mulligan for the New Zealand national side. Mulligan joined the Valiants on a free transfer from Sc**thorpe in January, but his contract runs out in the summer.
Mulligan could make his first start for six weeks tomorrow as midfielder Robin Hulbert is a doubt for the home game with Leyton Orient because of a shoulder injury. Paul Harsley is likely to switch from right-back to midfield, leaving Mulligan competing with Keith Lowe for the vacant spot."