Ronaldo is keen to leave United, just a year after completing a sensational return, as he wants to continue playing in the Champions League and secure his legacy as the competition's greatest goalscorer.
"I think United should let Cristiano Ronaldo go," Rooney wrote in his column for The Times. "It's not that Ronaldo can't play in a Ten Hag team. He can play in any team.
"Ronny will always score you goals. But my personal view is that United aren't ready to challenge for the title now, so the aim has to be to build a team that can win the league in the next three to four years, and you have to plan for that."
Ronaldo netted 24 times in all competitions last season, emerging as one of the few bright sparks in an otherwise forgettable campaign for United that saw them finish sixth under first Ole Gunnar Solskjaer and then interim boss Ralf Rangnick.
Ten Hag this week said he was happy to have a "top striker" like Ronaldo in the squad and looking forward to working with him.
Rooney, who is United's all-time leading scorer, added he was hopeful that Ten Hag would be able to establish a clear playing style at his former club.
"I couldn't work out what they were trying to do in terms of game plan, or see any patterns of play," Rooney said of last season.
"I think one of the big things you'll see from Ten Hag is him really trying to put his stamp on the playing identity."
Additional reporting by Reuters.