Other than the reason that there hasn't been any club that want Ronaldo I think if Man Utd sell Ronaldo the only option for Ten Haag for center forward is Martial, I am not saying that Martial is not good, but Ten Haag will need options or a substitute. And in this final weeks of transfer window finding a good striker that available is quite hard I think. Man Utd should have hunt for striker earlier, and if they actually want to let Ronaldo go, they should've done it earlier.
No, they will let Christian Eriksen play as center forward just like the previous matches against Brighton, Eriksen was mainly a Central Midfielder position lol. If not then Greenwood is the last choice since there's no player are better than him, or we may see Maguire will play on center forward? Manchester United is full of surprise and troll lol.
A serious suggestion for Manchester United, they need a huge reshuffling since only 1-3 players I'd say the performance are stable.
They also had Martial, which I know sounds like something they wouldn't want to hear after such a long break and I do not know when he will be ready to come back neither, maybe he is out for the season who knows? It's fully unknown, but he is a striker and he could play there if he ever comes back. They also have Rashford, who I know doesn't play striker naturally, but he could do that, he was tested there a lot and he proved that he could do it, would be harder for him to adjust now but he could do it.
So, between these 4 options, pick one and go with that if you want to, Rashford, Greenwood, Eriksen and if he gets better then Martial, all would be passable levels at least.