Louis van Gaal is desperate to strengthen his backline and will make it a priority when the transfer window reopens.
United's first choice target is Barcelona star Gerard Pique, who is valued around £20m.
Pique, 27, used to play for United before leaving for the La Liga giants in 2008.
Though United are willing to splash the cash to bring him back, the Barcelona star is reluctant to leave the Nou Camp - even for his old club.
United are considering a shock swoop for Real Madrid defender Raphael Varane if they fail in their bid for Pique.
Varane, 21, is regarded as one of the finest defenders in Europe but has struggled to secure regular football for Carlo Ancelotti's men.
He has found himself behind both Sergio Ramos and Pepe in the pecking order and admits the situation is frustrating.
Ron Vlaar is also on Van Gaal's radar.
The Aston Villa skipper, 29, is out of contract in the summer and could be available on the cheap as Villa don't want to let him go on a free.
But Vlaar has suffered injuries this season and there are doubts about his long-term fitness.
