The Spain Under-21 captain lavished praise on his compatriot and believes the national side can overcome a different group in this month's Euros
The Spain veteran fell out of favour with outgoing Santiago Bernabeu boss Jose Mourinho, who started choosing Antonio Adan over the club captain in December 2012.
The Portguese coach then refused to reinstall Casillas in the first team upon his recovery from a hand injury in 2013 as he stuck by January signing Diego Lopez.
De Gea, who left La Liga for Manchester United in 2011, told Europa Press: "At the end of the day, this is football and it’s quite normal for anyone to go through difficult moments - and it’s something you can learn from.
"We are talking about the best goalkeeper in the world and although I don’t know what went on with the coach, I do know that he will get over this – for the person he is and the quality he has. He will be back playing and playing well; and when he does, all of this will blow over."
De Gea was part of the Spain Under-21 side which were crowned European Championship winners two years ago and he hopes his experience can help lead them to glory again this month.
"We have had a couple of training sessions and everything seems to be going well," he continued. "As a group, we have known each other for several years and although we know that we have been thrown in a difficult group, we feel confident whilst realising that it is not going to be easy.
"As one of the older players, I can lend my experience to the younger members of the squad. I tell them that they may only get the chance to play this fantastic tournament once - twice if they are lucky.
"I just tell them to enjoy it and try to give the best of themselves because it’s such a special competition."
Spain will face Netherlands, Germany and Russia in Group B and De Gea thinks he and his team-mates have drawn the short straw against the most difficult opposition in the Euros.
The ex-Atletico Madrid shot-stopper added: "All of them are tough opponents, they are probably the three strongest teams in the tournament and all of them have players who also play with their respective senior sides so it’s not going to be easy for us."