We currently have one proven RHD on our roster in Carrier. I don't think signing another one blocks anyone.
For the sake of argument, let's say we sign Ceci and trade Matheson. Our pairs for next season could look something like this:
Guhle - Ceci/Carrier
Struble - Hutson
Xhekaj - Carrier/Ceci
In terms of depth chart, you have 4 LHDs (Hutson, Guhle, Struble, Xhekaj) and 4 RHDs (Ceci, Carrier, Reinbacher, Mailloux). It's hardly prudent to start the season with less than 8 NHL-capable defensemen anyway.
Nobody is blocked. Reinbacher or Mailloux has only to outperform one of Struble/Xhekaj and you put Hutson back on the left in such scenario.