Swansea acquired Bony in a club record deal last summer and saw their show of faith in his ability rewarded with 25 goals in all competitions.
The striker is being linked with Liverpool and Williams said: "It is very important he stays because he is a very good player, he has come back very focused, the best we have seen him. He is sharp in training and it would be a big boost to keep him.
"It would be bigger than getting a new signing - when you bring a new person in you are not sure how it is going to go."
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