The Middlesbrough winger is contracted to Boro until next summer.
Johnson will be able to sign a pre-contract deal with any club from January and has been linked with several Premier League clubs in recent times including Liverpool, Sunderland and Aston Villa.
Adam Johnson: “I have been linked with massive, massive clubs since I was 18. I thrive on it.
"When you get mentioned with these big, big clubs it boosts your confidence. Any player would like it.
"I tend to get on with my own game and whatever happens, happens.”