Rooney enjoyed great success at Old Trafford, where he became the club's all-time leading goalscorer while winning five Premier League titles and the Champions League among other silverware.
The 34-year-old, who left the club in 2017 following 13 years in Manchester, was also part of the last United team to win the Premier League in 2012-13.
Since then, United have struggled with only two runners-up finishes in the Premier League, while Ole Gunnar Solskjaer's fifth-placed side are 37 points adrift of runaway leaders Liverpool this term.
"It is going to take time. It is going to take another two or three years, I believe. They need to bring some players, they need to get rid of some players," Rooney said, quoted in several British newspapers.
"It's not going to happen [as a quick fix]. They've tried that with [Louis] van Gaal, with [Jose] Mourinho.
"If you look at Liverpool and what they've done, and Man City, you're not going to buy a team to go and challenge with them. You see with Liverpool - they've built that team.
"[Pep] Guardiola has gradually brought more players in and his way of playing.
"So United have to be a bit patient and try and build a team that will be able to challenge those two. The Manchester United fans need to be a bit patient with what is going on."
Rooney admitted to excitement at being drawn against his former club.
"I can't wait," Rooney told United's official website.
"Obviously, the draw, the one team I wanted was Man United. We were on the coach, actually, going to the hotel the night before a game when the draw was taking place and there was a cheer when Manchester United came out.
"It is a great draw for us, having United come to our ground and for the fans it is a great game to go to.
"For us as a team it is a great challenge for us to come up against Manchester United, to see if we can beat them. It is a challenge, but it is something we are all looking forward to."
United are three points adrift of Chelsea, who occupy the fourth and final Champions League spot after 28 rounds, ahead of Sunday's derby against Manchester City at Old Trafford.