Park moved to QPR from United over the summer.
"The great thing about Ji-sung Park is he's one of the best professionals we've had here," Sir Alex told ManUtd.com.
"He was truly fantastic, and particularly in big games. I loved playing him in the big games. His record against Arsenal, for instance, was fantastic."
"The Milan games over there and at Old Trafford, when I asked Park to play directly against Pirlo, are memorable. He never gave Pirlo a kick and he kept running off him which was brilliant too.
"Park had roles that only a disciplined and selfless player could have. That's exactly what Ji-sung was, selfless. He played for the team and whatever role you gave him, he would stick to that role. So he was a really wonderful, disciplined professional for us."