The defender joined Roma on-loan for the season, though is expected to sign permanently in January.
In a reply to a question at last week's Fans Forum, the club said: "The move was sanctioned in the interests of all parties.
"There is the possibility that the transfer could become permanent at the end of the loan period at around the fee Newcastle United initially paid for him.
"The club added that it had initially reached a verbal agreement for a replacement defender in the summer.
"Because the deal with AS Roma took longer than expected, the replacement transfer was no longer possible.
"The club's view was that the deal to allow Mapou Yanga-Mbiwa to leave was in the interests of all parties and, after taking the medium-to-long term view, the club let him leave."
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