United failed to retain the trophy after being pipped to top spot by City following Sergio Aguero's last-gasp winner against Queens Park Rangers on the final day of the season.
Sir Alex Ferguson's side squandered an eight-point lead in the final six games of the season to allow the Citizens the chance to gain ground on United in the run-in.
Phelan admitted he would like to forget certain results leading up to the last Sunday of the season, but conceded there have been times when it is impossible to forget.
"You have moments when you reflect," he said. "I wouldn't say you sit down and analyse and analyse and keep churning it over but there are moments when someone will say something or something else takes you back.
"Then you start to give it a bit of your time. It was dramatic, as dramatic as it has ever been, and very disappointing at the same time.
"In some ways, when you do look into it and start to think, you wonder whether the eight-point lead came at the right time. Did it settle us down too much?"
He continued: "Obviously there were games we should have won, that we were in control of, that we let slip.
"It was through lack of concentration in certain games, especially Everton, when we were at home and 4-2 up. It was there for a Manchester United win and it didn't happen.
"But you could also pinpoint other games; Wigan, losing a goal right on half-time against Manchester City, which changed the second half. The whole season just finished disappointingly flat."
