The former Barca captain says he lacked the support needed from the board at the time.
"I didn't receive the same support from the president that Xavi received . At that time I was sacked and I think we were nine points behind Real Madrid . And now we're ten points behind. To be successful you have to have the support of the president," he said.
Koeman is pleased, however, that this lack of support is not amongst Barça fans.
"When I walk around Barcelona I notice the affection of the people, it's not that it's less affectionate now because things haven't gone well. They remember me and will remember me as the man who scored the goal for the club's first European Cup . It's just that I didn't receive the support of the president."
Asked if it was the toughest moment of his career, he added: "I think so, but because Barça is my club. I had a great time as a footballer there, I have friends in the city, coaching the team was one of the dreams of my life... That makes everything more difficult and stressful."