Van Gaal was officially let go by United on Monday, two days after guiding the club to a record 12th FA Cup crown with a 2-1 win over Crystal Palace.
The Dutchman lasted just under two years in the role, and is expected to be replaced by former Chelsea boss Jose Mourinho in the coming days.
However, Blind - father of United defender Daley - believes the Premier League club were wrong to sack Van Gaal a year before his contract was due to expire.
"I'm closer to the situation than many other people," Blind told reporters at the Netherlands training camp in Portugal.
"And I can tell you that his departure came as a surprise. I feel for him. He doesn't deserve this.
"I've worked with him for so many years. I played under him for years at Ajax, I was his captain, I was his assistant at the last World Cup. He's a great coach.
"We all know that, as a manager, you will face the sack at some stage in your career. It happens to every manager.
"In this case I find it very disappointing. I know in which way this sacking was done and it's not the way it should go."