JuJu isn't a top tier WR but he is certainly in that second tier. Rookie WRs usually struggle. You can't count on a rookie contributing immediately. He is replaceable but it may not be easy. Most of the top WRs are gone already. If the steelers don't get a vet then they have to get lucky in the draft.
I don't think the team needs to get "lucky" since the draft pool at WR is deep. Also, a rookie this season won't need to be the X or indomitable, just really good as a rookie and somebody who develops into a WR defenses need to plan to stop. Juju came into a good situation in Pittsburgh, with Ben at QB, Bell at RB, Brown as the X receiver. He posted 58 receptions, 917 yards, 15.8 YPR, and 7 TD's as a rookie.
Steelers have two good receivers (Freiermuth and DJ), a clear #1 RB, and an OL that remains to be seen. Will be better with Daniels, Moore in his 2nd season, much improved depth and the draft yet to occur. I think a pick like Metchie III or Watson in the 2nd or even Pickens or Alec Pierce in the 3rd contribute a lot this season, work well as the Y or Z, force teams to use some double teams on them.
Watson is a physical freak with good hands, Metchie III is from what I have seen in looking at game film the best route runner in the class, Pickens has been haunted by injuries but his tools and hands are potentially elite, while Pierce is big, fast, physical, good hands who needs to work on fighting for the ball in traffic.
Getting one of those guys makes the WR corps better today and tomorrow and leaves the #20 pick for a DT, RT or CB.