The United boss stated at the weekend that it was Madrid's decision not to sell the Spain international and conceded that a deal to bring him to Old Trafford is unlikely.
However, compatriot Mata is refusing to give up on the idea that Morata could become his team-mate before the end of August.
"It looks tough to see Alvaro Morata in a Red Devils shirt with the arrival of Romelu," he told Cadena Cope. "We've signed an important player in attack and it's difficult, although you never know.
"Things can happen in the last few days, but I wish Alvaro the best."
Talk of Morata moving to United is unlikely to die down while speculation persists that Madrid are ready to reignite their interest in goalkeeper David de Gea.
A part-exchange deal involving the two players was mooted early last month, with Mourinho reportedly happy to consider allowing De Gea to leave.
Mata is confident that he will not be going anywhere, however, stating: "I think De Gea will stay."