Ralf Rangnick has been brought in on an interim basis to lead the team until the end of the season.
However, the Red Devils are facing huge pressure from Ronaldo to get the permanent replacement for Ole Gunnar Solksjaer right.
It is believed the 36-year-old is “considering his future” and could leave Old Trafford if he is not a fan of United’s next manager, even though he has a contract until 2023.
Sinclair has now told Ronaldo to keep quiet.
“I’m starting to think that Manchester City dodged a bullet by not signing Cristiano, I really think that,” he told talkSPORT.
“Know your role, know your role. You’re a footballer, you’re hired by the football club.
“Keep your mouth shut because at times that’s the best thing to do. He’s causing a lot of problems at the football club.
“Not just his lack of goals in recent times, but the fact that Ole’s gone and Michael went in and dropped him for a game.”