The Premier League leaders were seconds away from a third round exit at West Ham United before Van Persie scored a stoppage-time equaliser to secure a 2-2 draw.
Giggs, who had just headed a seemingly easy chance over from close range, sent a 60-yard pass over the home defence and Van Persie then beat West Ham goalkeeper Jussi Jaaskelainen from a position on the right of the penalty area.
“We missed some good chances, with headers over the bar and Shinji Kagawa had one blocked on the line, I thought it was not going to be our day.
“But you have to say the pass from Ryan Giggs, then the first touch from Robin and his finish were world class,” Ferguson admitted.
The replay will add to an already-congested fixture list for United, who will resume Champions League action next month, but Ferguson made it clear the FA Cup was still a competition he wanted to win.