The 30 year old England international told Inside United magazine: "When I signed my extension in May I said to David Gill: 'I hope this isn't the last one.'
"But I'll be 34 when this contract runs out and, if I'm honest, I'd have to be very lucky with injuries and keep myself in tip-top shape if I want to go beyond that.
"At the moment I'm just aiming to see out this term. If I'm still playing in the first team at 34, I'll be delighted.
"Maybe then I can say I've had a good innings."
The central defender has already enjoyed a successful career with three Premier League titles, a UEFA Champions League winner's medal and a League Cup triumph.
"I'm greedy. If I finish my career with what I've won now, I'll be very disappointed," he said.