Etheridge's signing comes in the wake of a dual weekend injury blow.
Rob Elliot picked up a hip injury in Saturday's 1-1 home draw against Tranmere, to be replaced by Ross Worner, who then suffered a dead leg.
Etheridge, who was on Chelsea's books as a schoolboy, is yet to make a first team appearance since making the short move to Fulham in 2006.
Source: BBC Sport
Source: BBC Sport