"These things happen but there is significant damage. Michael had worked hard to get back to a level where I was considering him for games.
"He was very much in my thoughts with Christmas coming up.
"It is going to be a difficult time for him but he has shown in the past real strength of character."
Since recovering from his long-term abdominal problem, he has made two appearances for City this season and scored against Scunthorpe in his side's 5-1 defeat of Scunthorpe in the Carling Cup.
Hughes had included Johnson on the bench in recent games, but his only Premier League action this season was as a 89th-minute replacement for Gareth Barry in City's 3-1 win over West Ham in August.
Johnson is a product of City's Academy and was captain of the team that finished runners-up in the 2006 FA Youth Cup.
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