The question I'm not seeing asked or answered is why would United want him?
They've got Mata, Lingard, James, Cavani, Martial, Rashford, Greenwood & Sancho already fighting for 3 spots. What exactly does Ronaldo add to them on the pitch? It's not a stop gap goalscorer that they need, they literally signed Cavani for that reason.
I find it hard to believe they'd sign a 36y/o on mega wages to purely stop him going to a rival - and if they do, so be it, likely doesn't change much for them on the pitch and hampers them going forward.