I still don't know what you mean.
Does the GOP now have to have one semator sit out whatever Johnson misses?
Why?
sounds crazy
NO, there's nothing obligatory about it. But it's commonplace for a senator who finds it inconvenient to get to a vote to ask for a pairing. If the other party refuses, then he may have to give up his plans and attend the vote. In Johnson's case, he might have to be wheeled in in a hospital bed and signal his vote in some way. But it's unlikely the Republicans would make him do that--apart from the fact that we still seem to have a 50-49 vote for the Dems.
I just googled for it, and couldn't find it for the US Senate, but here's the official definition for the Massachusetts Senate:
"Pairing of Votes - Procedure allowed in the Senate only, whereby a member, before the vote is taken, announces to the Senate that he or she has paired his or her vote with an opposing vote of an absent member. The two votes do not affect outcome of the final tally."