I wouldn't throw in the towel on Johansen just yet. If he is converted to a reliever and sticks as a closer, he can still prove worthy of his draft slot.
But, as squab mentions, it could just be a temporary thing. He has had control issues. Most times its a mental thing. Maybe they thinking getting him short stints with some success will help rebuild his confidence. He could still start a game or two before the season is over or he could return to the rotation next year.
Im good with him being a reliever though. With his power, if he can gain some confidence as a reliever, he could move quick and help at the big league level pretty soon.