The South Korean put the misery of almost nine months on the sidelines behind him when he came on for Cristiano Ronaldo in the Reds' 4-0 win at Sunderland.
"It was an important day for him. He came on and got more than half an hour under his belt so we are all pleased with him."
Park looked his usual busy self on the right side of midfield and showed no signs of his extended absence.
"He's a natural mover," Sir Alex added. "When his team passes the ball he moves - he's always had a great talent for that and today was another great example of that.
"He kept moving, he kept passing, he kept running behind the Sunderland defence. He gave us a lot of energy today and it was good to see."
