United have collected just one victory from their first five league outings, leaving them 12th in the standings and 10 points adrift of the leaders. Their early results have also effectively ended any realistic hopes of challenging for the title.
Rooney believes the squad was not strengthened sufficiently during the off-season, with the club bringing in only three outfield players. The Manchester United legend feels greater investment was required to address the team's shortcomings.
Pressure is now mounting on Michael Carrick, who was appointed as the club's permanent manager during the off-season. However, Rooney remains optimistic that his former team-mate can reverse United's fortunes.
The former striker believes Carrick has an opportunity to improve results following the international break and change the team's trajectory.
"There's always going to be pressure, Michael knows that," Rooney told talkSPORT.
"If you're not winning football games at Man United, then that pressure is there if you don't win one game.
"So, it hasn't been the ideal start to the season... there are certain things which I felt the club had to do before the window shut, in bringing players in... the squad is a bit weak."
He added: "He's got to find a way to get more and the best out of these players to get up the table after this international break... I think the break has come at the right time."
