It's always tough to say "so and so is undervalued" before the 1st preseason games, as momentum builds for the season, a lot of the undervalued guys become properly valued. Especially with how many reasonably intelligent people write about and analyze FF now.
I would say Ladarius Green is undervalued at TE though. He has top 3-4 upside in the Steelers offense.
Amari Cooper is very undervalued at WR, he lives next door to Derek Carr!
Moncrief is undervalued at WR as well, the Andre Johnson experiment is over and he is a physical freak with different talents than TY Hilton.
Brady is undervalued, I do think punting QB and getting him and then a Matt Ryan/David Carr type for 4 weeks is an optimal move. Thought this when Big Ben missed 4 games for being a rapist a few years back as well.
RB Latavius Murray is undervalued probably, but again by the time the season rolls around he may not be. RB is so fluid now, unless you are an elite talent they really don't let you touch the ball 300-350 times a season anymore.