Calderon sanctioned the purchase of Ronaldo back in the summer of 2009, after a world record fee of £80 million was agreed by the two clubs.
However, the Portugal captain appears to have become disillusioned with life in the Spanish capital, largely due to the club’s questionable transfer policy.
The likes of Gonzalo Higuain, Mesut Ozil, Xabi Alonso and Angel di Maria have departed the club over the past two years, a fact which has led to Ronaldo feeling unsettled at the club.
In an interview with TalkSPORT, Calderon said: “When I signed him in 2009, I remember he said he was grateful to Manchester United.
“He had a marvellous time there, he likes everything. It is a really great club.
“My impression is he is fed up with the policy of the current president (Florentino Perez).
“When he came with (Arjen) Robben, he thought he would be a good colleague for him in the team.
“(Gonzalo) Higuain was sold and he didn’t like that. Last year he thought it was a mistake to let (Mesut) Ozil leave and the last straw was selling Xabi Alonso and Angel di Maria this summer.”
Meanwhile, Sky Sports’ Spanish football expert Guillem Balague is of the opinion thgat Manchester United already have an offer lined up for the Portuguese forward.
He said: “I do believe Manchester United are preparing an offer, or have got an offer prepared, for Ronaldo and have had for months.
“I do believe he is thinking of going back, but there will undoubtedly be more on that later.”