The Spaniard was sold by Mourinho just six months into his second spell in charge at Chelsea in 2014 and many thought he would be leaving Old Trafford this summer.
Man United completed the signing of Armenia international Henrikh Mkhitarayan earlier this month for £26.3 million, but Mourinho insists there is space for Mata in his plans.
“I think since the first day, everything is clear between us,” Mourinho said.
“There is space for him. He is a talented player but I don’t promise places to anyone. I promise respect and I like him.
“So if he wants to stay and I think he wants – until this moment I don’t have one little sign that he wanted to leave so I think he wants to stay — then yes, there is space for him and yes, I think he can be useful to the club.”
Mkhitaryan put in a spectacular shift in the first-half against Wigan Athletic at the DW Stadium, but Mata went the perfect way about proving himself to the new boss with an assist for Will Keane.
Mata has scored 26 goals in 104 appearances for United since arriving from Chelsea and played a vital role for Louis van Gaal last season featuring in every game of the season.
It’ll be great for his confidence to hear Mourinho’s admiration of his talent, but he’ll have to work hard if he wants to have the same impact under the Portuguese boss as he has warned players he will not guarantee regular football for anyone at Old Trafford.