There are two schools of thought here. One is the the Texans have a propensity (recently) for drafting well and the rookie WR's will have a better chance to develop without another veteran WR. And two, they must go and get Johnson some help at WR mostly as protection against another injury to Andre.
Personally, I'm not comfortable with going into the season as is. On a team with a good WR corps., Walters and B. Johnson are #3 WR's. We can hope all day long that either Martin, Jean or Posey turns into that #2 or better yet #1B...but it will take some time.
Plexico Burress and Lee Evans are two players that the Texans should be looking at. They have both had success in the NFL recently in their careers. I'm really not sure why the Texans are not showing interest in at least one of them.