Johnson has struggled for form since his big money arrival from Manchester City.
"He has found it tough going," he acknowledged. "He belonged for a couple of years to a very, very big team (Manchester City). He was, I suppose, considered an impact player there, coming off the bench, and maybe sometimes when the team were losing coming in to make something happen.
"I know from my own experience as a player that sometimes that is not so bad because there is little expectation. The expectation here of Adam is that he beats four or five players, skips past them, and creates something or puts a great ball in. At the moment all those things we feel he is capable of doing are just not right there yet.
"But again, he has got the ability. It has all hit him so very, very quickly here.
"Honestly, he will eventually come good. In the course of time he will be a hit with the fans."