The future of Manchester United's first choice central defensive pairing is in doubt, with Rio Ferdinand struggling with injury, and Nemanja Vidic rumoured to be on his way out of Old Trafford.

Sir Alex Ferguson has rarely been able to start with the duo for much of this season, owing mainly to Ferdinand's persistent back injury, which looks set to hinder the England stopper until the end of his playing days.

The 31-year-old has rarely started back-to-back games this term, and has made several glaring errors for both United and England, amid fears that he may never rediscover the form that made him one of the world's top defenders.

And the United boss could soon lose his other defensive linchpin, with Vidic once again mooted to be planning a move to sunnier climes, with the Daily Mail claiming the Serbian ace will leave Old Trafford, possibly for Spain, in the summer.

Vidic and his wife Ana have apparently failed to settle in Manchester, and Sir Alex could be prepared to let the big Serb go at the end of the season in a bid to cash in on the 28-year-old before his contract runs down.

Talking to a United fanzine recently, Vidic refused to rule out the possibility of leaving the Premier League champions before the expiry of his current deal, which runs to 2012.

"To be honest, you never know what is going to happen. I’ll be 30 by the end of my contract."

"I can’t say what is going to happen in one year, two years’ time. At the moment I’m happy and I’m here. See what the club is going to say, what it is going to be."

Sir Alex has several well-admired young centre-halves on his wish list should his current pairing disband, with Palermo's Danish defender Simon Kjaer believed to have caught his eye with a number of glowing displays in Serie A.

Borussia Dortmund's 6ft 5in Serb Neven Subotic and Bolton's England hopeful Gary Cahill are also believed to be on Fergie's radar as he prepares for life without his defensive lynchpins at Old Trafford.