While he can’t be entirely written off just yet, things are not looking promising for Johnson’s 2015. He was a hot commodity coming in, and plenty of people (myself included) were feeling good about him.
It would appear that Teddy Bridgewater and Mike Wallace have a good thing going (both Wallace and Johnson had three targets in Week 2, though Wallace got 38 yards from them and Johnson only got ten), and Kyle Rudolph saw seven targets. I would feel comfortable stashing Charles Johnson if someone panicked and dropped him and I had a roster spot to play with.
If your roster can stomach it during the early bye weeks, the Vikings should hit their stride in Week 7 in a rematch with Lions. Until then, they face the Broncos, a bye week, and then the Chiefs in Weeks 4-6. Those are tough defenses. Johnson might be had free of charge before then as he is undeniably trending downwards, but do keep him in mind.