Former Bolton assistant manager Phil Brown was been impressed with the defender while he was on trial at the Reebok in 2003.
The Belgium international was seen as a replacement for Florent Laville, who had been ruled out until March 2004 following a knee operation.
Brown said at the time: "He does look the part.
"He is big in stature and is willing to learn.
"One of his major advantages is that he is all left-footed."
Leonard made his international debut in 1995 and won a championship medal with Monaco in France and the Belgian Cup with Standard Liege.
