But Mata says he has not seen anything in De Gea during their time with the Spanish national team to suggest that his reintegration with the rest of the squad will be a problem, and believes that he and Van Gaal will be able to continue their working relationship from where they left off last season.
"I've talked with him a lot. There have been hectic and stressful days but he is such a quiet, centred person, nothing changes," Mata told members of the British media ahead of Spain’s 2016 European Championship qualifier against Macedonia.
"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," Mata added.