Mourinho was part of the backroom staff during Van Gaal's time at Barcelona, and ahead of their clash on Sunday afternoon, the Portuguese coach has praised the United boss for the impact that he had on his development as a manager.
The 51-year-old told reporters: "He was important, he gave me a good opportunity to keep working at a high level.
"As a young coach to be working four consecutive years, three with Mr Bobby Robson, one with Louis, at a club like Barcelona was very important. If I had to chose some of the most important people in my career, Louis was one of them.
"I never hide the respect, the relation and the fact that he was important in my career. But after tomorrow, he is going to be my opponent."
Chelsea currently top the Premier League table, with United sitting 10 points behind in sixth position.