The 34-year-old's contract is due to expire in the summer and the centre-back is yet to open talks over a new deal.
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"I've spoken to him and, obviously at this moment in time, he's more concerned about getting an opponent for his testimonial game!" Sir Alex told reporters.
"He is working on that but we want him to stay and obviously I am sure his representatives will be discussing things with David [Gill, United chief executive].
"I would imagine that [staying] would be the best thing for him."