The centre-back, played primarily as a full-back by Alan Pardew, could now even join the Serie A side on a permanent deal.
But former Newcastle United defender Bernard has urged the Magpies to do everything in their power to prevent the Frenchman's full exit.
He says he 'can't understand' the decision to loan him out in the first place - and would 'never have let such a good player go.'
“I really, really didn’t agree with the club on that one,” the 34-year-old is quoted as saying by the Newcastle Evening Chronicle.
“I can’t understand it. I know Mapou and he has been the best defender in France for the past five years. He was regularly getting in the France squad and I was delighted when he came to Newcastle.
“But as soon as he came he was playing right-back, left-back and not enough in his proper position, which is centre-back. He is too experienced as a player to be playing right-back and left-back – he is not even left-footed.
“He just did not get a chance at Newcastle and when you see him getting in the France team and doing well for Roma, I just do not understand Newcastle’s decision to allow him to go out on loan.”
On whether Newcastle should stop Yanga-Mbiwa from leaving permanently, Bernard said: "Of course but I would never have let him go.
“We were not strong enough to allow such a good player to go out on loan. We have been struggling at the back all season and we allow such a good player to go out on loan. I don’t understand it.”
Yanga-Mbiwa has impressed since joining Roma, with the 25-year-old taking part both domestically and in the Champions League.