Derbyshire has struggled to make an impact since his summer loan move from Olympiacos but was in fine form on Saturday, grabbing a well-taken double in the 4-1 destruction of the npower Championship side at the New Den.
Blues' poor scoring record in the league - where they average just one goal a game - has led to McLeish step up his efforts to bolster his strike force by targeting Rangers striker Kenny Miller, who is expected to sign next week.
But Miller may face strong competition for a place up front from Derbyshire if he builds on his efforts and McLeish hopes the double strike will give the 24-year-old some much-needed confidence.
"Matt has been waiting for his opportunity," the Blues boss said after the game at Millwall.
"He played in a couple of Carling Cup games but looked a little short of fitness in the first few months, but those goals will do his confidence the world of good.
"Strikers thrive on confidence and he will benefit from that. He did well with his first goal and it was great to see it nestling in the back of the net.
"He is on loan until the end of the season. I have been waiting for the strikers to settle from the start of the season.
"We have worked hard and I cannot fault them with their efforts but they have been unfortunate with the finishing side of things.
"I believe there are goals in this side and Cameron Jerome and Matt showed that today.
"Cameron is normally worth 10-12 goals a season. He just needs a little break.
"Millwall had chances but the first goal settled us. If it had gone the other way it would have been game on and a complete change psychologically for our players.
"The play of Alexander Hleb and Jean Beausejour definitely made a difference for us with their threading of their through balls."