He doesn't need rest. And he is best keeper in the world. But TT since arriving always play Kepa in b2b games. Sometimes he plays before Cup game, sometimes after. Norwich at home is easier than Newcastle away so this is why people expect to see Kepa. It has nothing to do with Mendy really.
And don't forget without Kepa we would not win Super Cup and would be already out of Carabao Cup and state of the club and fans would be much worse so he deserves to play here and there more than many outfield players we see far often.