Ferguson made the move from Rangers to Ewood Park six years ago but was sidelined for a lengthy spell with a knee injury.
But McLeish is adamant Ferguson made a positive impact when he was able to play for Blackburn and can repeat that form for Birmingham.
McLeish said: "People say Barry had a rough time at Blackburn but I disagree with that.
"He had an horrific knee injury but, when he came back to full fitness, he was a fantastic player in the Rovers midfield.
"The big thing is the jury is still out on whether he can be a Premier League player because he didn't stay long enough at Blackburn.
"But I know from speaking to Walter Smith at the time, that he was getting fantastic reports about Barry Ferguson's performances at Blackburn.
"I know people said he was soon back up in Scotland because he hadn't done very well but that is nonsense."
McLeish added: "Barry has settled in quickly at Birmingham and played his part and, once he has got his top fitness and sharpness, I know he will be a big asset for us.
"You can see the way the lad controls the ball. He will take the ball in tight areas and manage to retain it.
"Others might not be as comfortable with that and Barry has to be careful choosing his passes and to not put some of our players into uncomfortable positions.
"But you've seen in the last two or three games that he is choosing his passes very carefully and keeping much valued possession."
Aston Villa 11/8, Birmingham 2/1, Draw 9/4