I talked to Sux about this. In my league, we decided is that it is not fair to be able to improve your team via injury. Meaning that you should not be able to put a player on IR, replace him, have that replacement become a stud then when the IR player comes back, drop a different marginal player. So, we had a rule that said, "when a player comes back from IR, you have to drop that player or the player you claimed as his replacement." We could use this for Covid or all IR.
(The first player you claim after an IR move is considered the Replacement Player, whether at the same time or later).