The 25-year-old has been strongly linked with a move to Portuguese giants Benfica.
Lennon has revealed that the club have written to one club to warn them off the former Newcastle goalkeeper.
He told the Scottish Daily Mail: “Fraser’s not for sale. We had no confirmed bids for him. We have actually written a letter to one club, just warning them off basically.”
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Since Guus Hiddink arrived at the helm, Chelsea have improved considerably after a tricky start to the campaign.
Newcastle United are staring down the barrel of relegation to the Championship for the second time in the past eight years.