Rojo joined United in a £16 million deal before the transfer window closed in August, which saw Portuguese winger Nani move in the opposite direction on a season-long loan.
However, the transfer was in danger of collapsing amid third-party ownership problems between Sporting and Doyen Sports, who owned 75 per cent of the player's rights.
The dispute led to Rojo walking out on Sporting, before former United and Argentina international midfielder Juan Sebastian Veron stepped in and urged his countryman to return to work.
"It was a huge surprise when I was told United wanted to buy me," said the 24-year-old, who is sidelined with a dislocated shoulder.
"I was in Portugal when my representative told me. He asked me to stay calm but I could not. I started living this dream.
"I could not think of anything else. I would call him every day, but when it seemed the transfer would not happen then I refused to work with Sporting.
"That was when Veron played his part in the process. He was like a strict father to me, scolding me.
"Veron called me one day when I had refused to train with Sporting and said 'Do not be stubborn. If Manchester United wants a player then they will not let you escape.'
"And so I went back... and the transfer went ahead."
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