The Egyptian winger is in danger of leaving as a free agent in the summer transfer window.
Salah is out of contract when this season concludes, with no agreement reached with the club as yet.
"We keep talking about him - he is unbelievable," said Richards after Salah scored twice and assisted in a 3-3 draw with Newcastle United.
"He doesn't even play well for 60 minutes but he's world-class. We talk about his contract, is he staying or is he going?
"They must keep him because he can do so many things others can't do.
"He doesn't have to be at the top of his game - not many people can score goals like that. Mo Salah is immense."
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