Gardner admits McLeish, who looks set to move to Aston Villa as Gerard Houllier's successor, has been a massive influence.
He said: "For me, Alex McLeish was great. He showed a lot of faith in me, signing me from Villa, and my career moved forward from there.
"It's a sad loss for Birmingham and it's not a good start to the summer.
"The club needs to bring in a manager with the kind of credentials and experience Alex McLeish has. Someone like that would fit the bill."