The 30-year-old featured in all but one of the east Londoners' Premier League fixtures last season, scoring six goals, but his future at the club has been called into question this summer.
"I can't say anything about Stewart as a player because everybody knows him," Karanka said.
"For me it is difficult to think about him because I know it is very difficult for me to get this type of player because he is a Premier League player, he has offers from other places but I am here to assemble a squad as good as we can."
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