But after failing to secure a move away from London Road during the transfer window he was brought back in by Gannon for his first game at the helm and was in the right spot to score the ninth-minute winner.
Posh are nine points adrift of safety at the foot of the Championship but Gannon is remaining positive.
He said: "Aaron justified his inclusion, not just with the goal but his overall enthusiasm.
"I came into the club this week and gave everyone a clean slate and he took his chance well.
"It was an easy decision for me to bring him back into the team. He has quality, he is a goalscorer and he has won us the game."
Gannon surprisingly played three at the back and captain George Boyd in a free role behind Mclean and Craig Mackail-Smith and was rewarded as all three combined for the winner.
Mackail-Smith chased a ball into the channel and did well to clip the ball back into the area and find Boyd.
The Posh captain had his back to goal but managed to lose his defender with a fine turn and coolly picked out Mclean in the six-yard box for a simple finish.
Gannon said: "The back three were immense. It was a new system for the players but they adapted well and deserved the win. The clean sheet was the priority today.
"We knew that QPR are statistically the worst side in the division in the second half of games so the plan was to get ahead and pick up a win and it worked out that way.
"We could have shown more control in attack and defence but overall it was a good, professional display.
"The priority was on defending but I didn't want to take away from our attacking strengths and the goal highlighted that. It was a fantastic move.
"We have quality at the top of the pitch and the three of them were first class."
QPR were poor as they suffered a fifth defeat in the last seven games and saw captain Mikele Leigertwood and defender Peter Ramage both sent off in the second half for late challenges on Tommy Rowe.
Leigertwood received a straight red for a high challenge while Ramage was unfortunate to be shown a second yellow card after catching Rowe late.
The defeat leaves Rangers marooned in midtable but boss Mick Harford refused to speak to the media after the game.