With Warnock's exit expected this summer, Sky Bet already had a market available on his successor, with the former Reading boss heading the market at 11/4.
Confirmation of the 64-year-old's departure, which came after a 2-1 defeat to Derby on Monday all but ended their promotion hopes, was followed by support for McDermott, prompting his price to come in to 11/10.
Neil Redfearn has been placed in charge on a caretaker basis but is deemed unlikely to get the position on a full-time basis at 16/1.
Sky Bet Football Trader Chris Spicer said: "McDermott had already been touted as the ideal replacement, having led Reading to promotion last season.
"However, the market remains wide open with Gus Poyet second favourite 5/1, while Owen Coyle (10/1) and Martin O'Neill (11/1) have also attracted some backing."