Calderon believes United lack the spending power to sign their former star.
When asked whether the Portugal flyer was happy at Real Madrid, Calderon told Talksport: "Ronaldo said he was sad at the beginning of the season. I think the relationship with the current president is not a good one, maybe because the president doesn't like the players he inherited.
"There are not many teams that could afford to pay him what he deserves and who can pay Real Madrid what they are asking for. There are three clubs - Manchester City, PSG and Chelsea. I don't think any other club is going to pay that amount of money for a player.
"They [Real] are going to negotiate the contract now and they will make a decision. It's something we will know in a few weeks."