Ferguson was a free-scoring attacking type of player during his days under McLeish at Rangers and he has effectively adopted more of a holding role at Blues.
He turned back the clock with his surging run and clinical finish to earn Blues a 1-0 home win over Nottingham Forest in the FA Cup third-round replay to secure an away tie with Everton.
McLeish said: "Barry has modified his game a bit. At Rangers he scored goals galore from midfield. In the treble-winning season, he got 18-20 goals which was amazing for a midfielder. He was very instrumental in us getting the victory with his performance, apart from his goal. He has made a big contribution since he came here."
He added: "He made a lot of forward runs and, in that Rangers team, we had fantastic players who could play you into good positions and Barry is a good timer of runs.
"We've asked him to change a little bit since coming to Birmingham and Lee Bowyer has taken the lion's share of the goals.