Wickham struck twice to see of Capital One Cup opponents MK Dons.
Johnson said: "He's had a tough time here, especially the last two months of last season. He's worked so hard to get himself in good shape and get back among the goals.
"I know the manager has worked individually with him and that's starting to show.
"He's got everything there. He's big, strong and has now scored two goals.
"If he keeps his confidence up, he'll score a lot more for us."
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