The Mysterio family, including Murphy, were missing from WWE TV for most of December and Rey Mysterio recently revealed he and Dominik tested positive for COVID-19 on December 11 before that night's episode of Friday Night SmackDown.
Rey and Dominik have since returned to WWE TV and reignited their feud with King Corbin, but Murphy hasn't appeared on the blue brand since defeating The Lone Wolf on December 4.
According to Wrestling Observer's Dave Meltzer, Murphy's return was affected by Steve Cutler's positive COVID-19 test. WWE reportedly planned for the Mysterio family's next storyline to be Rey, Dominik and Murphy against Corbin, Steve Cutler and Wesley Blake. Cutler tested positive for COVID-19 after attending a New Year's Eve party, however, and Vince McMahon decided not to bring The Knights Of The Lone Wolf back to TV. Cutler has since been released by the company.
With The Knights Of The Lone Wolf no longer alongside Corbin, WWE decided to just use Rey and Dominik in the feud and currently "there's no spot" on TV for Murphy.
When the former Raw Tag Team Champion will return to WWE TV is currently unknown.