Forssell struck the first blow after only seven minutes and after we'd dictated the rest of the first half, was clinical in front of goal as the home side took advantage of us pushing forward for goals.
Sebastian Larsson curled home a free-kick after 55 minutes, Forsell made it 3-0 four minutes later and completed his hat-trick 10 minutes from time.
McLeish said: "I'm really pleased for Mikael. I've had him in and out of the team since I came here but the lack of goals in previous weeks meant I had to push his experience back into the team.
"He was merciless in front of goal. He could have had four or five goals but his link-up play was also good. He had a terrific game.
"He's had a lot of injury problems and when they are of that magnitude it is going to have some effect.
"It is a case of whether he can play 40 games a season but, when he is as hot as that, we have to wrap him in cotton wool."
McLeish praised the overall performance of his side as they gave their hopes of avoiding relegation a massive lift.
He said: "We have deserved more from previous games at St Andrews and this time we got our reward.
"It was worth the wait. It was a fantastic show. The players' work rate in every game cannot be faulted but this time we also added the finishing touch."
Birmingham City boss Alex McLeish singled out hat-trick hero Mikael Forssell for praise after his side's 4-1 win at St Andrews.