McLean, who is on the transfer list, returned to the Posh side and duly obliged by scoring the only goal of the game at London Road to give Gannon a winning start to his managerial reign with Posh.
"Aaron justified his inclusion, not just with the goal but his overall enthusiasm," Gannon said after the 1-0 win on Saturday afternoon.
"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.
"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."