Martinez blamed the setback on the knee injury Ivan Ramis suffered at Fulham last week, which led to a defensive frailty, with Steven Fletcher moving onto 10 goals for the season with an impressive first-half double.
But Martinez is adamant that, after eight years in the top flight, no-one at Wigan is resting on their laurels.
"We never take anything for granted," he said.
"If we had done that we would never have had eight seasons at this level.
"Today we weren't good enough defensively for 30 minutes. We need to rectify that.
"Ivan Ramis' injury last week cost us today's points. That is something I need to correct."