Reports last month stated the Red Devils had taken up their 12-month option on the Spain midfielder, whose current deal expires at the end of the season.
United, however, are continuing to drag their heels, with concerns his present £190,000-per-week salary will simply put off prospective buyers in terms of them recouping a future fee of any real significance.
That has already alerted Mata's first club Valencia to the attractive possibility of landing him on a free transfer in the summer, having made 120 first-team appearances for Los Che before his switch to Chelsea back in 2011.
Mata has significant business interests in Manchester but has previously spoken of his love for Valencia, who are understood to still hold very good relations with the 29-year-old's agent father, Juan Snr.
United boss Jose Mourinho wants to conduct a major overhaul of his squad at the end of the campaign and knows the club may first have to move out some of their higher-earning peripheral squad players to help him achieve it.