The Black Cats beat Danish Superliga side Midtjylland 1-0 in midweek thanks to an Adam Johnson goal but the performance left the Italian boss frustrated with his team.
"I'm not happy," said Di Canio, quoted in the Daily Mirror. "I was disappointed with the attitude. The attitude is crucial.
"We can talk about tactics during the season or technical mistakes that can be improved by practice, but the attitude is the thing you don't know.
"Some of my players are new, so I don't know. I would not be arrogant enough to think I can discover their attitude in two hours of playing.
"It was a big opportunity for all the players to show we are ready to start the season strong, but some of them failed to complete the mission.
"I have to make sure they change their mentality otherwise we will have a problem. I didn't see the performance I wanted to see."
Sunderland kick off their Premier League campaign against Fulham on August 17th and Di Canio was disappointed to see levels drop from the Asia Trophy, where his team beat Tottenham.
He added: "We have 10 days to go and it is the last real game before the start of the season - I'm not happy because it was an important game for me.
"I would have preferred to lose the game and see a better attitude and more improvement in our play, rather than win and take one step back from what we showed in Hong Kong. So I didn't enjoy it.
"I know that when you are engaged in a process of building something, you can have one step forward and two back - or two forward and three back.
"But I expected a bit more. I didn't enjoy watching my players play - not all of them but a few of them."