Sir Alex: Man Utd performance not good enough
Nani doubled that fourth-minute advantage before the break as United looked set to stroll home.
But after losing Ryan Giggs to a hamstring injury, United collapsed in a six-minute spell after the interval, with Patrice Evra scoring an own goal before Edwin van der Sar's calamitous mistake presented Somen Tchoyi with an open goal.
"It is an inexplicable result and obviously frustrating," said Ferguson.
"I suppose there are two ways of looking at it. If we were in the middle of the league we could say we were still undefeated. For us it is not good enough."
United have now drawn five times this season, including three on the trot in the Premier League.
They have also tossed away two-goal advantages on three occasions, which cost them precious points here and against Everton, with Dimitar Berbatov coming to their rescue to snatch victory against Liverpool.
"Sometimes it is better to be one goal in front than two. You keep your focus a bit more," said Ferguson.
"But what can you say about their second goal? Edwin van der Sar has probably been playing for 25 years and he was probably in primary school the last time he made a mistake like that.
"You have to just wipe it off the slate. It is not a defining result in terms of where the league is going to end up but we cannot keep doing that."