Flitcroft accepted his side had been second best on the day as they fell to a convincing 4-0 defeat but felt they were harshly treated by the dismissal of Danny Mayor, who was shown a straight red card in first-half stoppage time.
O'Shea scored with an unstoppable strike from 25 yards and Marc Richards pounced on a Roberts flick-on to finish before Dan Gardner marked his debut with the fourth in the 86th minute.
Flitcroft was full of praise for the way Chesterfield played but was less impressed with the decision to end Mayor's afternoon early.
"When I look at where I want to take this team, I need look no further than what the plan is here and what they are trying to do," he said.
"It was incredible quality from a team of very good players managed by a good manager and some of the forward play at times was breathtaking.
"I'm in admiration of the way they beat us but I felt for the sending-off that Danny had pulled out of the tackle. We have been fairly beaten but it's not a 4-0 and that's what I'm disappointed about. I really wanted to be competitive here and our fans have been cheated of that right.
"O'Shea's goal was as good as you would see this afternoon and we have been beaten by more quality which is something for us to aspire to. But we didn't keep the ball well enough and we gave arguably the best team in this league too much of the ball."