The system he plays in at United vs Spain are so radically different. One is a more defensive system and even then he's expected to face shots outside the box or from corners more frequently. With Spain he could go for a good 30 mins without being called into action much due to Spain always seeing more of the ball.
Casillas with the national team at his peak despite not being good with the ball at his feet, he was always Alert and ready to deal with danger when needed. It's the same with Valdes at Barca as well. With De Gea I don't have any confidence in him and he seems a lot more limited than I realized.