Atsu is playing for Hatayspor in Turkey but his whereabouts are unknown after an earthquake devastated the south of the country and northern Syria.
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There had been claims that the 31-year-old Ghana international had been found, but his club have since denied those reports.
Atsu played for a number of Premier League clubs, including Chelsea and Newcastle, while he also spent time under current Magpies boss Howe at Bournemouth.
"It's hugely worrying," the Englishman said.
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"I really, really enjoyed working with Christian, a great lad, great player. Our thoughts are with him and his family.
"We hope for some good news, we hope he's OK, but we're really concerned for him and his welfare this week."
"It's been difficult because there have been conflicting stories coming out about his whereabouts, so it's been very tough. I can't imagine how his family are feeling.
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"But from the bottom of our hearts, we wish him well and hope there's a positive ending."
Atsu's Ghana teammate and new Nottingham Forest forward Andre Ayew also discussed the situation, and was clearly very affected.
Ayew also played in Turkey for Fenerbahce, as well as linking up with Atsu for the Ghana national team.
"I'm very, very sad, very down," he said.
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"For such a beautiful country, beautiful people, there are no words to describe how I feel.
"With the Christian situation it's been difficult, trying to speak to each other, trying to get hold of his family, we're praying that he will be OK, the whole of Ghana is praying for him.
"It's difficult to speak about because you can't really show what you feel, which is inside of you, when you think about it and know people involved, it's hard, it's sad, it's difficult."
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"It's hard to know what to think, you realise in this world anything can go in one second.
"We're behind him and we believe until the end and hopefully he will be OK and his family will be relieved and the whole of Ghana will be relieved."