The duo featured regularly for Louis van Gaal last season, making a combined 69 Premier League starts.
However, according to Sky Sports News, Mourinho has already deemed both players to be surplus to his plans at Old Trafford.
Mourinho, then of Chelsea, sold Mata to United in January 2014, supposedly because he did not believe that the playmaker had the pace, workrate or tactical nous to operate within his preferred system.
Meanwhile, the report adds that Blind does not have the physical stature to play in the heart of the defence under Mourinho.
It is believed that the pair will be allowed to leave as soon as the Portuguese coach secures the services of direct replacements.