But the Welsh players have made it clear they hopeÂ the Australian, who took over from Mike Ruddock last month, remains as their coach.
And Williams believes Johnson is "one of the best coaches I have worked with".
"I know I speak for the boys that we don't want to see him go," said 35-times capped Williams.
"He has got personal reasons for whatever decision he is going to make, and we are going to have to respect that.
"He is a great guy and we just hope he stays. I personally want him to stay. Â
"Among others he has been at the heart of Wales's success and he's brought so much to the team - I have learned so much from him.
"He has an all-round knowledge of the game. He can tell you anything about rugby, not only now but about something that happened about 100 years ago.
"He is so knowledgeable and his intelligence of rugby makes the game a lot easier for the players.
"I certainly haven't worked with anyone like him before and he certainly one of the best coaches I have worked under."