Sir Alex Ferguson is becoming increasingly worried over his first-choice defensive partnership of Ferdinand and Vidic, reports the MEN.
Ferdinand only played 19 times last season for United because of leg and back problems, and also missed the World Cup because of a knee ligament injury suffered in the first training session in South Africa.
The 31-year-old is hoping to be fit to face newly-promoted Newcastle at Old Trafford when the 2010/11 Premier League season kicks off on August 14.
But that seems ambitious given his progress, and there are also fears that Vidic may be on the way this summer.
The Serbian hardman has yet to settle a new Old Trafford deal and Real Madrid have constantly been linked as Jose Mourinho looks to strengthen the Galacticos at the back.
Ferguson is already planning for the future having signed rookie centre-back Chris Smalling from Fulham - and although Jonny Evans is also fighting for a starting role, the United gaffer wants to splash some cash on Cahill.
The 24-year-old has been tipped as a future England international but has yet to win a full cap.
Cahill has admitted he would be open to a move to a big club, saying: "Being realistic, if a big team wanted me to come and play for them, then you have got to look at it. It may be a right decision for both sides.
"I'm not saying for one minute I want to leave Bolton. I have loved my time here and the club has done brilliantly for me."
