The Ospreys have bought Waikato Chiefs' New Zealand A scrum half James Nutbrown out of his contract a year early to bring him to the Liberty Stadium to push Phillips.
Nutbrown was born in Canterbury and was part of the Crusaders squad that won the 2005 Super 12 title. He has represented New Zealand at all age groups as well as A Team level and had been hotly tipped in his home country for a call-up to the All Blacks squad for the autumn international series.
Having come to prominence with the Crusaders, where he shared scrum-half duties with Marshall, Nutbrown spent the last three seasons with the Chiefs, where he has been involved in a keenly contested battle for a starting place with All Black Brendan Leonard.
He has also represented Bay of Plenty in the Air New Zealand Cup during this time, and was appointed captain for the Steamers 2008 campaign.
"The esteem in which Jamie is held in his home country is clear in the way that the NZRU were so keen to keep hold of him. We've worked long and hard over the last month or so to negotiate an early release from his existing contract as we believe he will be a real asset to the Ospreys," said Ospreys director of rugby Andrew Hore.
"Jamie has a wealth of Super 12/14 experience with both the Chiefs and the Crusaders, as well as playing for New Zealand A. He has an excellent work ethic and sets very high standards on and off the pitch.
"His arrival will give us another option at scrum half, providing real competition for Mike Phillips, and increasing the strength in depth of our squad. As well as looking to push Mike for the No 9, I also see him playing a big role in the continued development of young Rhys Webb."
Phillips underwent knee reconstruction during the summer and is set to return to action in October. Last season he ousted 2005 Lions scrum half from the starting position in the Welsh Six Nations side to become the senior partner for the first time.
At his best he is seen as a leading candidate for a place in Ian McGeechan's Lions party to go to South Africa next summer.