If you've read our fantasy baseball coverage in the past, you know we don't rely solely on a pitcher's ERA to analyze performance. An earned run average can be affected by numerous factors, some out of the pitcher's control. If a pitcher avoids allowing baserunners, he's going to generally avoid allowing runs. Sometimes bad luck intervenes, and in the case of Johnson, it appears he was one of the unlucky ones, though he was a big winner. While some can disparage Johnson for this, his stats can be used the other way as well, to prove he wasn't bad at all.
I use WHIP when examining pitchers more than ERA, because it's a better indicator of whether a pitcher was successful. A guy can allow a four-run inning in a lot of different ways, but four runs are still four runs. Did he allow seven baserunners to do it, or was it a single, walk, hit by pitch and grand slam? Also, ballpark factors affect ERA more than WHIP, as well as defense.