De Gea was all set to move to Madrid on the final day of the Spanish window but late paperwork meant the deal fell through.
The keeper hasn’t made a single appearance for Man United so far this term but there have been calls for him to start against Liverpool following the international break.
Mata revealed that he often spoke to De Gea during the “hectic and stressful” days of the window.
“I’ve talked with him [De Gea] a lot. There have been hectic and stressful days but he is such a quiet, centred person, nothing changes,” Mata said.
“The last two years David has been the best player in our team. His saves have earned us a lot of points. I have no doubt this will be a very good year too.
“I don’t think there’ll be a problem with the club, the coach or the fans. That’s what I perceive. The fans love him very much, he’s highly regarded by the club and the coach.
“I think it will be the same. Herrera and I will continue saving on petrol!”
Mata, De Gea and Herrera have something of a bromance going on at Old Trafford and it’s good to see that they can continue it for another year. De Gea would no doubt have been mentally affected by the debacle but he must focus on getting back to his best for United, starting with the Liverpool game this weekend.