Billy Sharp, left unmarked in the box, put United ahead in the 18th minute and then missed a penalty just before the break after Jacob Greaves brought him down, although McCann felt that the Blades striker was offside.
He had no issue with the validity of United’s second or three goals from John Egan, who headed home from two corners on opposite sides of the pitch from Conor Hourihane to give the Blades a 3-0 buffer after just past the hour mark.
Keane Lewis-Potter’s consolation ensured that City avoided the ignominy of setting a club record of seven league games without a goal, but McCann had bigger concerns at the final whistle.
McCann, whose side have not won since the opening day of the campaign on August 7 and are without a home victory this term, said: “I am disappointed. We came up against a very good team who will get better and better and go from strength to strength. But for us, we cannot keep coming in and keep saying we have had good spells in games.
“Some of the play and opportunities were good, but we need to do better. Simple as that.
“I take the responsibility for it as I am the manager and we have to do better.
“We have lost three or four games this season on set-plays and that is a kick in the teeth for us and it was the same again.
“We were 1-0 down and were still in the game. But the first (goal from Egan) was disappointing. They are an experienced team. Billy Sharp got hold of Mallik (Wilks) and let Egan round around the back. It is good play from them, but disappointing from us.
“We understand there has not been many free-kicks and penalties given this season for those situations this year, so we need to get better at that and understand that and not let people block us.
“It was a frustrating moment for us and then to concede from another set-play was unacceptable.
“Our best markers in the box let us down for a few goals. We have got to learn and grow up.”
McCann, who felt that the officials made the wrong call in also ruling out a late goal for offside from Tyler Smith, who had earlier set up Lewis-Potter’s leveller, revealed that Greg Docherty and Tom Huddlestone missed the game through hamstring injuries.
He said: “It is disappointing for Greg. He felt his hamstring on Friday. I don’t think ir will be too long and I think maybe he will play next week.
“I left Tom out on Tuesday as I wanted to try and get him on board for this game and unfortunately, he picked up a hamstring injury in training in a light session, which is frsutrating for us.
“But it is not too bad. It is a ten-day or two-week job.”