Atwell is going to be behind Nacua for one really important reason. Puka gives the Rams positional flexibility that Atwell doesn't. Puka can run jet sweeps, he can block, but most of all he can play all 3 WR positions should Jefferson or Kupp be out. Atwell is strictly a Y and Z receiver who isn't going to provide any blocking. Puka plays like a fast TE and can threaten all 3 levels.
Atwell is going to be a good rotational player at Y and Z but unless someone is hurt IMO that is the limit of his abilities which is enough.
Atwell needs to nail the return specialist to help his cause. Xavier Smith is basically the same height as Atwell and equally fast. He also has experience returning kicks. If Smith flashes as a receiver, Atwell will have competition especially if Smith going to return kicks.