A day after United had sealed their own spot courtesy of a last-gasp extra-time win against Premier League Fulham, Forest also had to work hard - taking their place in the last 16 courtesy of Jamie Mackie's superb finish and Darius Henderson's late strike.
Scotland international Mackie reacted first when the ball broke to him on the edge of box after 18 minutes, guiding his shot into the top corner to give the visitors the lead.
Preston enjoyed plenty of the ball from there, with veteran striker Kevin Davies going close on more than one occasion to bringing them back into the tie.
The 36-year was perhaps lucky to stay on the pitch after escaping what looked to be a wild, lunging first-half challenge on Djamel Abdoun without even a yellow card, while after the break he fired just wide of Dorus de Vries' goal.
The former Bolton forward climbed highest just after the hour mark but could not direct his header from Lee Holmes' cross on target, and he then had strong claims for a penalty turned down when Greg Halford appeared to block his shot with a hand as Preston failed to get the goal their endeavour deserved.
With a minute left their luck was summed up when Joe Garner's shot deflected onto the post as the hosts failed to find a way through, before substitute Henderson went down the other end to seal the result, latching onto a loose ball to fire home.
