A day after Manchester City had dropped points at big-spending QPR, United were expected to stroll to victory against Southampton.
But even after overcoming the loss of an early goal to lead 2-1 at half-time, United encountered massive problems in the second half as Southampton took control and were highly unfortunate to gain no reward for their efforts.
As his side start a run of fixtures mainly against clubs in the lower half of the table, Ferguson believes such games merely highlight the difficulties his players will face unless they have the right attitude.
"The effort Southampton put in on Wednesday is a reminder to us that you get three points for every game, not just for beating teams at the top of the league," the Scot said ahead of Saturday's trip to Fulham.
"We've had great examples of that over the years. Last year we lost to Blackburn Rovers at home. You could quite easily say that cost us the league.
"I don't expect my players to be complacent, but Wednesday is obviously a great reminder."
After such a gruelling game on an awful pitch, Ferguson is intending to make changes for the Craven Cottage encounter.
If Saturday's FA Cup victory over Fulham is any guide, United should encounter few problems at Craven Cottage.
Martin Jol's men were swept aside last weekend with barely any resistance after falling behind to Ryan Giggs' early penalty.
However, the Cottagers are a different proposition on home soil and will approach their task with confidence following their own midweek victory over West Ham.
"Fulham's home record is very good and has been for a few years," Ferguson added. "They are one of the few clubs who get full houses every week.
"It's a healthily run club and you always get a good atmosphere, which is important. Hopefully we get the result we want because we are at the stage in the season where every game becomes important."