United have lost three games so far this season and Rooney fears that more defeats could be on the way if United carry on playing like they did in the early stages of the win over QPR.
Rooney feels that the improving needs to start at the back, tightening up a defence that has let in 18 goals so far this term.
"Clean sheets are the foundation we hope for, we know we have players who can score goals and win us games," Rooney told Sky Sports.
"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."
Source: United Mad