Goals from Andy Butler and John Marquis saw Rovers through in a low key affair at The Valley.
While delighted his side had advanced in the competition, McCann was frustrated with the lack of tempo in Rovers' play against a much-changed Charlton side.
'I thought we were disappointing to be honest,' McCann said.
'I was disappointed with the first half. We weren't at our usual standards today.
'A little bit better in the second half but I thought we were sloppy.
'I said to the boys at half time and after the game that I will always be honest as long as I'm the manager of the football club.
'I didn't think we reached our usual standards but we've won the game, we're into the next round and that was the most important thing today.'
McCann believed a combination of factors could have resulted in a lacklustre performance but says he expected Rovers to overcome them.
'The atmosphere in the stadium, it was almost like a friendly game in the first half,' he said.
'It was dead, from both teams. There was just no real tempo in the game.
'I don't know whether it's a mindset thing, whether we knew they'd made a lot of changes.
'But I said to them that the boys who came into the Charlton team would want to try to impress their manager today. We knew it'd be difficult.
'I didn't think we had that spark about us today, that energy, that aggression in our play that we've had for most of the season.
'Even at 2-0 up, Charlton had opportunities to score a goal. That's disappointing and frustrating from us because we should be managing the game better.'