Fox has missed extensive time this season with a broken foot. The move to sign Moore is a fail safe as Ryan Donahue, who is still dealing with a quad injury. Donahue was injured trying to fill in for Jason Hanson earlier this week, whose own injury status is still up in the air. Hanson is still battling a knee injury and has been limited in practice this week.
Fox has not played this year, as noted by Pride of Detroit:
Dave Birkett reports that Fox was limping at practice yesterday. Perhaps he reinjured his foot, or perhaps the Lions simply needed to clear a roster spot and it was clear Fox wouldn't be on the field in the near future.
It is unknown is Malone will definitely fill in for Donahue on Sunday or if he is insurance.