The former Barcelona winger said while United dragged their heels over his transfer, Chelsea were more decisive and the prospect of living in London also influenced his decision.
Pedro admitted he did not like the way Van Gaal treated his compatriot Victor Valdes, who has been reportedly training alone and stripped off a squad number after being accused of refusing to play for the under-21 side.
"It's true that I don't like that treatment for my colleagues, especially for Victor, who has been a reference as a team-mate and as a professional," the-28-year-old Pedro told Spanish newspaper Marca.
"I don't believe it's fair but it didn't influence my decision. I just saw that Manchester didn't take that move forward, that they were taking too long. Chelsea were more decisive.
"I had an important chat with Jose Mourinho and he said many things that I liked to hear. That, combined with the possibility to live in London, was what made me decide," the Spain international added.
Pedro has gotten off to a flying start with the Blues, scoring one goal and providing two assists in his first two games, although Chelsea have managed only four points from as many games.
He will be hoping to add to his goal tally when his side make the trip to Goodison Park to face Everton on September 12 in what promises to be a heated affair after Chelsea's public pursuit of John Stones during the summer transfer window.
Image: Pedro of Chelsea.
Photograph: Reuters