Justin Marshall is undecided whether to prolong his playing career

Justin Marshall could go into coaching when he hangs up his boots, but only if he can work on his temperament.

The Ospreys scrum-half will take over from injured Wales number nine Mike Phillips for Sunday's Heineken Cup quarter-final away at Saracens.

The ex-All Black, 34, has just over a year left on his contract and may opt to coach when he decides to retire.

"It's been mentioned to me by people I respect. They feel I'd have plenty to offer coaching," he told Sport Wales.

Interview: Justin Marshall talks to Sport Wales

"But I really have to grapple with that, temperamentally, the game still frustrates me too much.

"Aspects of my personality at the moment would be detrimental to a team environment, rather than a positive factor.

I never had a great relationship with Byron Kelleher within the All Blacks squad. He would really get on my wick. Justin Marshall

"My immediate reaction to some situations is anger and frustration.

"I don't think that good coaches, or people who know the game well, have that reaction. Their first thought is 'how can I teach that player to make a better decision?'

"To be a good coach you have to be a very good man-manager and I'm not sure I'm suited to that yet. But maybe I can work on it."

Marshall is still open-minded over whether he will seek another playing deal at the end of next season.

His decision could rest on whether or not the Ospreys lift the Heineken Cup in this season or next.

Marshall's intense rivalry with Phillips at the region has been pin-pointed for the Kiwi's return to world-class form this season, but the former Canterbury Crusaders player disputes the suggestion.

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"Some people have told me that the rivalry has spurred me on, but personally I disagree.

"Throughout my career I've always had people snapping at my heels so there's been nothing different this season.

"I've always had to be aware I needed to be at the top of my game. Mike Phillips' arrival didn't send any new warning signals.

"The rivalry with Mike has always had an edge to it, but that's because of elements within my personality.

"I never had a great relationship with Byron Kelleher within the All Blacks squad. He would really get on my wick.

"He was forever telling me he was going to take my shirt and he deserved to have it. I always felt he should stop talking about it and go out and prove it."