The Italian club has a view to a permanent deal at the end of the season with some reports suggesting the Giallorossi will purchase him at a reduced rate.
However, Charnley has revealed that there were plans in place to sign a replacement for Yanga-Mbiwa who will certainly not cost the Toon after they invested in him for just 46 matches.
"The move was sanctioned in the interests of all parties," said Charnley.
"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 Newcastle board added: "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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