Yes, you certainly have the ability of stopping a player move, especially if it was something like a loan with you paying the obscene wages he's on. Now the question is where was he going to and MU said no? Which club with ambitions matching his and enough money in the bank wanted him, and did bring an actual offer on the table? All of those he was rightly or wrongly linked with publicly said feck no, not interested. Some of them many times. The only real offer was from Al Hillel club where Ronaldo was certainly not going. Napoli's was a piss take.
You're also right when saying unless you're directly taking part in the transfer deals, no one would know with certainty what really happened. What I do know, though is if Ronaldo was still even half of the player some people here think he is, the offers would've come, real ones, and he'd be gone. You're seeing right now what his actual level is. You can cling onto the latest excuse in date that it's because he missed the pre-season, which was entirely his own doing, but I wonder how much longer it will hold. He's not immortal and age is not something you can win against.
He shouldn't have come back to United or the PL, it burned him out and shortened his days at the top level. Now chances are big that it will screw up his last WC.