By Frank Taddeo The Pittsburgh Steelers and star wideout WR Diontae Johnson agreed upon a two-year, $36.71 million extension. With Johnson’s contract now completed, the issue of determining his fantasy football value takes center stage due to the inexperience at the Pittsburgh quarterback position. In early drafts, the former Toledo standout is being drafted in the late third/early fourth round. He’s currently listed as the 19th wide receiver on Athlon’s fantasy football cheat sheet. Related: Fantasy Football Wide Receiver Rankings Johnson has steadily improved his production in each of his th…