United came from behind to win 2-1.
"I thought we were poor. Over the last four years we have allowed the (Anfield) crowd to get at us a little bit," he said.
"They gave fantastic support to their team and they dominated. We played better in the second half but it was against 10 men and Paul Scholes, Michael Carrick and Ryan Giggs' experience got us through.
"No doubts about the penalty though, it was reckless. We haven't played well here for the past four years and we can do better but at least we got a result. It is a turning point for us."
Ferguson was also pleased his side had broken their duck from the spot after three misses - one from Van Persie himself - this season so far.
"It wasn't easy for Robin. He took it well. It was the way he envisaged he would," added the United boss.