United are a point clear of champions and rivals Manchester City - who remain undefeated, whilst Sir Alex Ferguson's side have lost three times already.
Rooney admits that they need to address their form, starting at the back, as United have conceded 18 goals so far this season.
"Clean sheets are the foundation we hope for, we know we have players who can score goals and win us games," he said.
"If we can get a clean sheet, then I'm sure we can get them at the other end and win games.
"As a team, we've played some good stuff at times but we've also ground results out as well. To lose three games this early is something we need to put right.
"We're lucky that we've won all the other games and not drawn any. It means we're still in a good position in the league.
"So we need to make sure we are full focused now and try to kick on, try to go on a good run."
Their failure to keep clean sheets has also seen them concede first 12 times in their last 20 games, including their last match on Saturday as they came from behind again to beat Queens Park Rangers 3-1.
"I don't know why it is but it's happening too many times," Rooney said.
"We need to just go out now and try to get the early goal. As soon as we get a goal, the team settles down and the game settles.
"We tend to take control of the game. But more often than not we've conceded first and made it hard for ourselves to try to get back in the game then go on and win it."