Sir Alex signed Park from PSV Eindhoven in 2005, and at the time United were accused of signing the player purely for his commercial value.
But Park is now one of United's most trusted performers and usually features in all the club's vital matches.
Park stooped to score the goal which gave United their 2-1 win over Liverpool at Old Trafford, and Sir Alex said: "I didn't think that (Park was bought just to sell shirts). When I went to see him play in those Champions League semi-finals for PSV Eindhoven in 2005 I thought this is a player who understands football.
"He is intelligent and disciplined and he can play different positions. I had no issues about that at all. Someone is always going to take a runner on something like selling shirts. But you could say that about every player we have signed.''
Park said: "It was a great feeling to score against Liverpool. Derby matches are very important games, so to score the winner in one is a fantastic feeling. Doing it in front of the Stretford End was unbelievable.
"I want to score more goals for United and it doesn't matter who they are against. I know I should score more goals. I feel good scoring against the big teams like Arsenal, Milan and Liverpool, but I want that experience more often.''