The Reds had taken a first-half lead thanks to Jadon Sancho's maiden home goal, but with Matt Crooks levelling things up midway through the second half, it was Chris Wilder's side who progressed to round five, thanks to their 8-7 victory in a marathon penalty shootout.
With United quickly returning to action in just three days' time, Rangnick believes the only aspect missing from his side's performance was the ability to find the back of the net.
''I think we have played well in the last couple of weeks, in the league and also today in the first half,'' began the 63-year-old.