The Red Devils have won just one Premier League game from five played and Michael Carrick's team have already been eliminated from the Carabao Cup.
Carrick was heavily backed in the summer transfer window with over £150M spent on midfield trio Youri Tielemans, Andrey Santos and Carlos Baleba - with the latter yet to feature due to injury.
United have looked particularly uncertain in defence - with a 4-0 UEFA Champions League win over Sabah their sole clean sheet of the campaign so far - and Rooney has raised some serious questions over the current state of play.
"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.
"It hasn't been an 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."
United are next in action back in the Premier League at home to struggling Tottenham on October 10th.