I would say the reason he was undrafted is because no one trusts Baltimore's passing attack past Todd Heap. Lewis is the main option on first and second down, and the Ravens normally play close defensive-minded games which require ball control through the run. Johnson is a good receiver, but the question is will he actually get the touches and yards necessary to be a good fantasy starter.
Another reason why is because I only see 14 players for your team (assuming you have a full roster). That's only 140 players taken in total. A 12 team league with 15 players would draft 180...Johnson would most likely be taken in the latter system, but maybe not in the 10 teamer.
I think the bottom line is to ask yourself, "Is he better than any of the WR's on my team?" All the receivers you have now are pretty good and/or the same as Johnson. But, if you think he's going to bust out, then you might consider acquiring him.
The One, the Only, the Incomparable Mercer Boy.
My My YouTube