The pair missed Tuesday's 3-3 home draw with Basel in the Champions League as Rooney was recovering from a hamstring problem and Hernandez from a dead leg but attacking matters were not Ferguson's chief concern on Friday.
"We're too cavalier at the moment," the manager told a news conference on Friday. "We have to balance both things and that's why it's called a team."
Unbeaten United have scored 22 goals in six league games so far and are top of the table on goal difference from neighbours Manchester City.
The Premier League champions scored two quickfire goals against Basel on Tuesday and appeared to be cruising to victory but a shaky defence cost them as they found themselves 3-2 down before Ashley Young's last-gasp goal rescued a draw.
"People say our performance was careless and we under-estimated our opponents and maybe there's an element of truth in that," Ferguson said.
"I certainly wasn't happy, even in the first half I could sense we were too relaxed and could have lost three goals in the first half.
"We could have scored goals as well but were too cavalier. We're attacking in the way we're playing at the moment and people can enjoy it but we have to do better defensively."
He was encouraged by the fact that his team always look like creating and scoring, which will be helped by the return of Hernandez and Rooney, who has already scored nine league goals this season and is back from injury sooner than expected.
"Wayne and Javier Hernandez will be fit for tomorrow. They trained yesterday very well and I was pleased with that," said Ferguson, who had originally said Rooney could face a few weeks out when he was injured before last week's game at Stoke City.
"[Defender] Jonny Evans also comes back, hopefully that will help us in the defensive part as we're certainly changing the back four at the moment."
Ferguson was full of admiration for Saturday's visitors Norwich, who were promoted at the end of last season and sit ninth in the 20-team table.
"Norwich have made a terrific start. You look at Norwich and see full houses every week. There's something happening down there, a vibrancy about the place," Ferguson said.
"They're playing with a great sense of purpose and you can see progress every week."
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