Johnson, his stellar summer singing, has failed to spark into life.
O'Neill admitted: "If you analyse his form, it's not surprising.
"He had a cameo role at Manchester City and was playing with great players, coming on for 15-minute spell and not having to worry about defending because others in the City side would take care of that for him.
"He has come to us and is an integral player in our team and he has to adjust to that. It takes time.
"He did well when Stewart Downing left Boro and got his move to City. Adam has more than enough natural talent to be successful."